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.     

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.     

Maximizing the purification of the activated glucocorticoid receptor by DNA-cellulose chromatography.

With heat treatment (20 degrees C for 30 min), the glucocorticoid-receptor complex becomes 'activated' and undergoes an increase in affinity for DNA. A two-stage procedure was used to separate sequentially the rat liver glucocorticoid-receptor complex from proteins with high and low affinity for DNA. DNA-cellulose column chromatography of unheated cytosol resulted in the retention of DNA-binding proteins, but not the unactivated receptor complex. Heat treatment of the column eluate resulted in increased affinity of the receptor complex to DNA, and chromatography on DNA-cellulose then yielded receptor complex free from proteins with low affinity for DNA. Removal of DNA-binding proteins during the first chromatographic step was critically dependent on ionic conditions and the ratio of cytosol chromatographed to DNA-cellulose. A purification of 11000-fold (85% yield) was achieved by this procedure. The partially purified receptor complex was taken up by rat liver nuclei.     

Relationship between glucocorticoid receptor steroid-binding capacity and association of the Mr 90,000 heat shock protein with the unliganded receptor

Treatment of rat liver cytosol with hydrogen peroxide (H2O2) or sodium molybdate (MoO4(2-)) inhibits thermal inactivation of glucocorticoid receptor steroid-binding capacity at 25 degrees C. Dithiothreitol (DTT) prevents the stabilization of receptors by H2O2. Heating (25 degrees C) of immune pellets formed by immunoadsorption of L-cell murine glucocorticoid receptor complexes to protein-A-Sepharose with an anti-receptor monoclonal antibody (BuGR2) results in dissociation of the M 90,000 heat shock protein (hsp90) from the steroid binding protein. Such thermal-induced dissociation of hsp90 is inhibited by H2O2. Pretreatment of immunoadsorbed receptor complexes with the thiol derivatizing agent, methyl methanethiosulfonate (MMTS) prevents the ability of H2O2 to stabilize the hsp90-receptor interaction. These data suggest a role for hsp90 in maintaining an active steroid-binding conformation of the glucocorticoid receptor.     

Evidence that removal of an endogenous metal that stabilizes the untransformed glucocorticoid receptor in cytosol allows ligand-independent receptor transformation

Cytosol preparations contain an endogenous heat-stable factor which stabilizes the glucocorticoid receptor in its untransformed, non DNA-binding form. Elution of a partially purified preparation of this stabilizing factor through a metal chelating resin (Chelex-100) leads to the loss of its ability to inhibit temperature-mediated transformation of the receptor. Sodium molybdate mimicks the ability of this endogenous metal to stabilize the untransformed receptor, and it too is adsorbed by Chelex resin. When an L-cell cytosol preparation containing the glucocorticoid receptor is passed through a column of Chelex-100 resin and then incubated at 15 degrees C, the receptor is rapidly transformed to the DNA-binding state, regardless of whether it is steroid-bound or not. In contrast, whole cytosol containing endogenous metals is transformed to the DNA-binding state only when the receptor is both steroid-bound and exposed to elevated temperature. these data suggest that a metal (or metals) may be involved in conferring the property of ligand-dependency to the transformation process.     

The heat-stable cytosolic factor that promotes glucocorticoid receptor binding to DNA is neither thioredoxin nor ribonuclease

Treatment of rat liver cytosol containing temperature-transformed [3H]dexamethasone-bound receptors at 0 degree C with the sulfhydryl modifying reagent methyl methanethiosulfonate (MMTS) inhibits the DNA-binding activity of the receptor, and DNA-binding activity is restored after addition of dithiothreitol (DTT). However, transformed receptors that are treated with MMTS and then separated from low Mr components of cytosol by passage through a column of Sephadex G-50 have very little DNA-binding activity when DTT is added to regenerate sulfhydryl moities. The receptors will bind to DNA if whole liver cytosol or boiled liver cytosol is added in addition to DTT. The effect of boiled cytosol is mimicked by purified rat thioredoxin or bovine RNase A in a manner that does not reflect the reducing activity of the former or the catalytic activity of the latter. This suggests that the reported ability of each of these heat-stable peptides to stimulate DNA binding by glucocorticoid receptors is not a biologically relevant action. We suggest that stimulation of DNA binding of partially purified receptors by boiled cytosol does not constitute a reconstitution of a complete cytosolic system in which the dissociated receptor must associate with a specific heat-stable accessory protein required for DNA binding, as has been suggested in the "two-step" model of receptor transformation recently proposed by Schmidt et al. (Schmidt T.J., Miller-Diener, A., Webb M.L. and Litwack G. (1985) J. biol. Chem. 260, 16255-16262).     

Transformation of glucocorticoid-receptor complex oligomers to DNA-binding forms in the absence of monomerization

The contention that transformation of steroid-receptor complexes is represented by dissociation of receptor oligomers was tested by comparing sedimentation and DNA binding properties of glucocorticoid-receptor complexes from HeLa cell cytosol under several conditions. Transformation of glucocorticoid-receptor complexes could be induced by heat, and/or salt treatment of cytosolic extracts, but not by dilution. Heat-induced transformation of receptor complexes was also confirmed by DEAE-cellulose chromatography. Analysis of cytosolic extracts showed that sedimentation and DNA binding properties of glucocorticoid-receptor complexes did not correlate. Both oligomeric and monomeric receptor complexes, in fact, were found to be either transformed, or untransformed, depending on the treatments cytosolic extracts underwent, before being subjected to analysis. We then concluded that release of glucocorticoid receptor monomers cannot account for their transformation to a DNA-binding form in vitro, and suggested that exposure of positive charges on the surface of receptors in the course of transformation occurs in some region of the glucocorticoid receptor which is not involved in interactions between the proteinaceous components of oligomers.     

Inactivation of rat liver glucocorticoid receptor by molybdate. Effects on both non-activated and activated receptor complexes.

When freshly prepared glucocorticoid-receptor complex from rat liver cytosol was incubated at 23 degrees C in the presence of sodium molybdate, its subsequent binding to isolated nuclei, DNA-cellulose and ATP-Sepharose was blocked. In addition, binding to these acceptors by cytosol receptor complex fractionated with (NH4)2SO4 was also blocked by incubation of the complexes with 50 mM-sodium molybdate. However, molybdate had no effect on the binding of activated receptor complexes to ATP-Sepharose. Molybdate was also effective in extracting the nuclear- and DNA-cellulose-bound glucocorticoid-receptor complexes in a dose-dependent manner. Molybdate appears to exert its effects directly on the receptor by interacting with both non-activated and activated receptor forms.     

DNA-binding and non-DNA-binding forms of the transformed glucocorticoid receptor

In this work we have probed the mechanism responsible for two non-DNA-binding states of the mouse glucocorticoid receptor. In the first case, transformed receptors were treated with hydrogen peroxide. It is known that oxidizing agents promote the formation of disulfide bonds in the glucocorticoid receptor, but it has not been determined what domains are involved in any disulfide bond formation that leads to inactivation of DNA-binding activity. We show here that hydrogen peroxide inhibits DNA-binding by the 15-kDa tryptic fragment containing the DNA-binding fingers with the same concentration dependency as it inhibits DNA-binding by the uncleaved receptor. This suggests that all of the effect of peroxide is on sulfhydryl groups within the zinc fingers. After dissociation (transformation) of cytosolic heteromeric glucocorticoid receptor complexes, only a portion (40–60%) of the dissociated receptors can bind to DNA-cellulose. We show that the 15-kDA tryptic fragment derived from the portion of transformed receptors that do not bind to DNA is itself competent at DNA-binding.     

Glucocorticoid receptor phosphorylation, transformation, and DNA binding

Glucocorticoid receptors were isolated by immunoadsorption from cytosol of L cells that were cultured for 18 h in the presence of [32P]orthophosphate, and the phosphorylation state of the receptor was examined before and after transformation to the DNA-binding state. Temperature-mediated transformation of the glucocorticoid receptor under cell-free conditions results in no change in receptor size or degree of phosphorylation. When cytosol containing transformed receptors is incubated with DNA-cellulose, 30-50% of the receptors are able to bind to DNA and the remainder do not bind to DNA. Both the heated receptors that bind to DNA and the receptors that do not bind to DNA are phosphorylated to the same degree. When intact cells containing 32P-labeled receptors are incubated for 2 h at 0 degree C with triamcinolone acetonide and then for 20 min at 37 degrees C in the presence of the hormone, 80% of the receptor becomes tightly associated with the nucleus in a manner that is both temperature-dependent and ligand-dependent. Approximately 80% of the nuclear-bound receptor is extracted with 0.4 M NaCl. Both the cytosolic receptor from cells incubated at 0 degree C and the salt-extracted nuclear receptor from cells incubated at 37 degrees C have been resolved by immunoadsorption to protein A-Sepharose with the BuGR1 monoclonal antibody and sodium dodecyl sulfate-polyacrylamide gel electrophoresis, followed by immunoblotting and autoradiography of the immunoblots. In addition, direct measurements of the amounts of 32P contained per unit of receptor protein were performed for receptors transformed both in the intact cell and in cell-free lysates. The results demonstrate that the untransformed receptor and the nuclear-bound transformed receptor are labeled with 32P to the same extent.     

Degradation without apparent change in size of molybdate-stabilized nonactivated glucocorticoid-receptor complexes in rat thymus cytosol

We have investigated the stability of the [3H]dexamethasone 21-mesylate-labeled nonactivated glucocorticoid-receptor complex in rat thymus cytosol containing 20 mM sodium molybdate. Cytosol complexes were analyzed under nondenaturing conditions by gel filtration chromatography in the presence of molybdate and under denaturing conditions by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate. When analyzed under nondenaturing conditions, complexes from fresh cytosol and from cytosol left for 2 h at 3 degrees C eluted from gel filtration as a single peak of radioactivity with a Stokes radius of approximately 7.7 nm, suggesting that no proteolysis of the complexes had occurred in either cytosol. When analyzed under denaturing conditions, however, whereas the fresh cytosol gave a receptor band on sodium dodecyl sulfate-polyacrylamide gel electrophoresis at Mr approximately 90,000 (corresponding to the intact complex), the cytosol that had been left for 2 h at 3 degrees C gave only a fragment (Mr approximately 50,000). This fragment, just as the intact complex, could be thermally activated to a DNA-binding form. Proteolysis of the receptor could be blocked by preparing the cytosol in the presence of EGTA, leupeptin, or a heat-stable factor present in the cytosol of rat liver and WEHI-7 mouse thymoma cells. From these results we conclude: (i) 20 mM molybdate does not protect the nonactivated glucocorticoid-receptor complex present in rat thymus cytosol against proteolysis under conditions which are commonly used for cell-free labeling of the receptor, and (ii) the demonstration of a Stokes radius of approximately 8 nm for the nonactivated glucocorticoid-receptor complex is not sufficient to indicate that the receptor complex is present in its intact form.     

Evidence that pH induced activation of the rat hepatic glucocorticoid-receptor complex is irreversible

The possible reversibility of pH induced activation of the glucocorticoid-receptor complex was studied. Generally, this was accomplished by activating rat liver cytosol at pH 8.5 (15 degrees C, 30 min), and then returning it to pH 6.5 for a second incubation (15 degrees C, 30 min). Activation was quantitated by measuring the binding of [3H]triamcinolone acetonide [( 3H]TA)-receptor complexes to DNA-cellulose. When cytosol was incubated at pH 6.5, only 4.1% of the [3H]TA-receptor complexes bound to DNA-cellulose. However, 39.2% of the complexes bound when the cytosol was pH activated. When pH activation was followed by a second incubation at pH 6.5, 47.0% of the steroid-receptor complexes bound. Thus, according to the DNA-cellulose binding assay, pH induced activation was irreversible. In order to visualize both activated and unactivated [3H]TA-receptor complexes during this process, diethylaminoethyl (DEAE)-cellulose chromatography was performed. When cytosol was incubated at pH 6.5, only 19.6% of the [3H]TA-receptor complexes were eluted in the activated form from DEAE-cellulose. However, 67.5% of the complexes were eluted in the activated form when cytosol was pH activated. When pH activation was followed by a second incubation at pH 6.5, 74.9% of the steroid-receptor complexes were eluted in the activated form. Thus, DEAE-cellulose chromatography also showed that pH induced activation was irreversible. This is the first known report that the combination of DNA-cellulose binding and DEAE-cellulose chromatography have been used to study pH induced activation of the glucocorticoid-receptor complex. By these criteria, we conclude that in vitro pH induced activation is irreversible.     

Activation of the human glucocorticoid receptor: evidence for a two-step model

The relationship between glucocorticoid receptor subunit dissociation and activation was investigated by DEAE-cellulose and DNA-cellulose chromatography of monomeric and multimeric [3H]triamcinolone acetonide ([3H]TA)-labeled IM-9 cell glucocorticoid receptors. Multimeric (7-8 nm) and monomeric (5-6 nm) complexes were isolated by Sephacryl S-300 chromatography. Multimeric complexes did not bind to DNA-cellulose and eluted from DEAE-cellulose at a salt concentration (0.2 M KCl) characteristic of unactivated steroid-receptor complexes. Monomeric [3H]TA-receptor complexes eluted from DEAE-cellulose at a salt concentration (20 mM KCl) characteristic of activated steroid-receptor complexes. However, only half of these complexes bound to DNA-cellulose. This proportion could not be increased by heat treatment, addition of bovine serum albumin, or incubation with RNase A. Incubation of monomeric complexes with heat inactivated cytosol resulted in a 2-fold increase in DNA-cellulose binding. Unlike receptor dissociation, this increase was not inhibited by the presence of sodium molybdate. Fractionation of heat inactivated cytosol by Sephadex G-25 chromatography demonstrated that the activity responsible for the increased DNA binding of monomeric [3H]TA-receptor complexes was macromolecular. These results are consistent with a two-step model for glucocorticoid receptor activation, in which subunit dissociation is a necessary but insufficient condition for complete activation. They also indicate that conversion of the steroid-receptor complex to the low-salt eluting form is a reflection of receptor dissociation but not necessarily acquisition of DNA-binding activity.     

Elimination and reconstitution of the requirement for hormone in promoting temperature-dependent transformation of cytosolic glucocorticoid receptors to the DNA-binding state

Cytosols contain a heat-stable, chelatable, anionic, molybdate-like factor that stabilizes glucocorticoid receptors in a heteromeric complex with hsp90 (refers to the 90-kDa heat shock protein) and inhibits their transformation to the DNA-binding state (Meshinchi, S., Grippo, J.F., Sanchez, E.R., Bresnick, E.H., and Pratt, W.B. (1988) J. Biol. Chem. 263, 16809-16817). In this work, we demonstrate that removal of this factor by passage of L cell cytosol through the metal-chelating resin Chelex-100 makes the glucocorticoid receptor unstable, thus markedly facilitating both its dissociation from hsp90 and its transformation to the DNA-binding state. In normal cytosol, both temperature-mediated dissociation of hsp90 and temperature-mediated receptor transformation are hormone-dependent events. In the Chelex-treated, metal-depleted cytosol, however, temperature-mediated dissociation of hsp90 and receptor transformation occur very rapidly in a manner that is no longer hormone-dependent. When boiled L cell cytosol is added to the metal-depleted receptor system, the hormone dependence of both temperature-mediated dissociation of receptor from hsp90 and receptor transformation to the DNA-binding state is reconstituted. Like boiled cytosol, molybdate stabilizes the receptor complex and inhibits its transformation in metal-depleted cytosol, but it does not reconstitute the hormone dependence of the system. These results support the proposal that an endogenous metal anion interacts with the glucocorticoid receptor to stabilize it in the heteromeric, inactive, non-DNA-binding state in cytosol and that binding of the hormone promotes conversion of the receptor to the DNA-binding state through an effect on this metal anion center.     

Modulation of DNA binding of glucocorticoid receptor by aurintricarboxylic acid

Effects of aurintricarboxylic acid (ATA) were examined on the DNA binding properties of rat liver glucocorticoid-receptor complex. The DNA-cellulose binding capacity of the glucocorticoid-receptor complex was completely abolished by a pretreatment of receptor preparation with 0.1-0.5 mM ATA at 4 degrees C. The half-maximal inhibition (i.d.50) in the DNA binding of [3H]triamcinolone acetonide-receptor complex [( 3H]TARc) was observed at 130- and 40 microM ATA depending upon whether the inhibitor was added prior to or following the receptor activation. The entire DNA-cellulose bound [3H]TARc could be extracted in a concentration-dependent manner by incubation with 2-100 microns ATA. The [3H]TARc remained intact under the above conditions, the receptor in both control and ATA-treated preparations sedimented in the same region in salt-containing 5-20% sucrose gradients. The action of ATA appeared to be on the receptor and not on DNA-cellulose. The DNA-binding capacity of ATA-treated receptor preparations could be recovered upon exhaustive dialysis. The treatment with ATA did not appear to change the ionic behavior of heat activated GRc; the receptor in both control and the ATA-treated preparations showed similar elution profiles. Therefore, ATA appears to alter the binding to and dissociation of glucocorticoid-receptor complex from DNA. The use of ATA should offer a good chemical probe for analysis of the DNA binding domain(s) of the glucocorticoid receptor.     

Partial purification of the glucocorticoid receptor from rat liver: a rapid, two-step procedure using DNA-cellulose.

Glucocorticoid receptor from rat liver was purified 1800-fold by a rapid two-step procedure using DNA-cellulose. The procedure is based on increasing the affinity of the glucocorticoid-receptor complex for DNA by heating the complex. During a first chromatography step, unheated glucocorticoid-receptor complex is separated from cytosol proteins that bind to DNA-cellulose with high affinity. During a second chromatographic step, heat-treated glucocorticoid-receptor complex is separated from proteins with low affinity for DNA. The partially purified complex is functionally competent in that it is taken up by isolated rat liver nuclei.     

Identification of a macromolecular inhibitor of glucocorticoid-receptor complex activation in rat liver cytosol

We have identified an endogenous regulator of the glucocorticoid receptor following fractionation of dialyzed rat liver cytosol on DEAE-cellulose. The macromolecular regulator, purified approximately 20-fold as judged by Lowry-reactive material, inhibits activation of glucocorticoid-receptor complexes when assayed by DNA-cellulose binding and by chromatography on DEAE-cellulose minicolumns. In addition the active DEAE-cellulose fraction stabilizes the unoccupied glucocorticoid receptor against heat inactivation. Evidence is presented that the observed inhibition of activation by the active DEAE-cellulose fraction is not due to concentration of cytosolic proteases or RNA. The inhibitory molecule in the active fraction is not stable to heating at 90 degrees C (15 min) and is partially inactivated at 45 degrees C (15-60 min).     

Chromatographic separation of nontransformed and transformed glucocorticoid-receptor complexes from rat liver cytosol. Use of tungstate in the purification, inactivation, and resolution of receptor forms

Aliquots of rat liver cytosol glucocorticoid-receptor complexes (GRc) were transformed by an incubation with 8-10 mM ATP at 0 degrees C and were compared with those transformed by an exposure to 23 degrees C. The extent of receptor transformation was measured by chromatography of the samples over columns of DEAE-Sephacel. The ATP-transformed complexes, like those which were heat-transformed, exhibited lower affinity for the positively charged ion-exchange resin and were eluted with 0.12 M KCl (peak-I): the nontransformed complexes appeared to possess higher affinity and required 0.21 M KCl (peak II) for their elution. As expected, the receptor in the peak-I exhibited the DNA-cellulose binding capacity and sedimented as 4S in sucrose gradients. Peak II contained an 8-9S glucocorticoid receptor (GR) form that showed reduced affinity for DNA-cellulose. Presence of sodium tungstate (5 mM) prevented both heat and ATP transformation of the GRc resulting in the elution of the complexes in the region of nontransformed receptors. When parallel experiments were performed, binding of the cytosol GRc to rat liver nuclei or DNA-cellulose was seen to increase 10-15 fold upon transformation by heat or ATP: tungstate treatment blocked this process completely. The transformed and nontransformed GRc were also differentially fractionated by (NH4)2SO4: tungstate-treated (nontransformed) receptor required higher salt concentration and was precipitated at 55% saturation. In addition, the GRc could be extracted from DNA-cellulose by an incubation of the affinity resin with sodium tungstate resulting in approximately 500-fold purification of the receptor with a 30% yield. These studies show that the nontransformed, and the heat-, salt-, and ATP-transformed GRc from the rat liver cytosol can be separated chromatographically, and that the use of tungstate facilitates the resolution of these different receptor forms. In addition, extraction of the receptor from DNA-cellulose by tungstate provides another new and efficient method of partial receptor purification.     

Evidence that the hormone-binding domain of the mouse glucocorticoid receptor directly represses DNA binding activity in a major portion of receptors that are "misfolded" after removal of hsp90.

After dissociation of cytosolic heteromeric glucocorticoid receptor complexes by steroid, salt, and other methods, only 35-60% of the dissociated receptors can bind to DNA-cellulose. The DNA-binding and non-DNA-binding forms of the dissociated receptors have the same Mr and are phosphorylated to the same extent (Tienrungroj, W., Sanchez, E. R., Housley, P. R., Harrison, R. W., and Pratt, W. B. (1987) J. Biol. Chem. 262, 17347-17349). The basis for the different DNA-binding activities is unknown, but the DNA-binding fraction of the receptor has a more basic pI than the non-DNA-binding fraction (Smith, A. C., Elsasser, M. S., and Harmon, J. M. (1986) J. Biol. Chem. 261, 13285-13292). We have separated the non-DNA-binding state of the receptor from the DNA-binding state and then cleaved it with trypsin and chymotrypsin. We find that the 15-kDa tryptic fragment derived from the non-DNA-binding state of the dissociated receptor is fully competent in binding DNA, whereas the 42-kDa chymotryptic fragment containing both the hormone-binding and DNA-binding domains does not bind DNA. Trypsin cleavage of the molybdate-stabilized untransformed receptor also yields a 15-kDa fragment that is fully competent in binding DNA. Reducing agents do not restore DNA-binding to the non-DNA-binding fraction of the receptor and the hormone-binding domain can be separated from the DNA-binding domain on nonreducing gel electrophoresis. These results argue that the two domains are not linked by disulfide bridges, and they are consistent with the proposal that there are two least energy states of folding after dissociation of hsp90. A significant portion of the receptors is "misfolded" in such a manner that the steroid binding domain is directly preventing DNA-binding activity.     

In vitro modulation of rat liver glucocorticoid receptor by urea

We have examined the influence of urea on the properties of the rat liver glucocorticoid receptor (GR). A 1-h incubation of hepatic cytosol with 1-3 M urea at 0 or at 23 degrees C caused a progressive decrease in the steroid binding efficiency of GR. Urea treatment of cytosol incubated with 20 nM [3H]triamcinolone acetonide caused transformation of glucocorticoid-receptor complexes (GRc) and resulted in an increase in the binding of GRc to DNA-cellulose and ATP-Sepharose. The transforming effect was maximal with 2.5 M urea at 0 degrees C for 1 h, and it caused a shift in the rate of sedimentation of the 9 S untransformed GRc to a 4 S form, similar to that observed upon incubation of the cytosol GRc at 23 degrees C. This 9 to 4 S transformation could also be observed in the presence of Na2MoO4. The Stokes radii of the GRc eluted from a Bio-Gel-A-0.5m agarose column were determined to be 5.9 and 4.9 nm in the absence and presence of 2.5 M urea. The aqueous two-phase partitioning analysis revealed a significant change in surface properties of GR following urea treatment; the observed partition coefficient values (cpm upper phase/bottom phase) were 0.022, 0.208, and 0.60 for GRc, GRc + 23 degrees C, and GRc + 2.5 M urea, respectively. Furthermore, the urea treatment rendered the GRc less negatively charged, forcing their appearance in the flow-through fractions of a DEAE-Sephacel column. These results suggest that urea is a potent in vitro modulator of the physicochemical behavior of GR, influencing both the steroid binding and the process of receptor transformation.