MOLECULAR AND CELLULAR BIOLOGY, Sept. 1998, p. 4949–4960
0270-7306/98/$04.0010
Copyright © 1998, American Society for Microbiology. All Rights Reserved.
Vol. 18, No. 9
HSP90 Interacts with and Regulates the Activity of Heat Shock
Factor 1 in Xenopus Oocytes
ADNAN ALI, STEVEN BHARADWAJ, RUTH O’CARROLL,
AND
NICK OVSENEK*
Department of Anatomy and Cell Biology, College of Medicine, University of Saskatchewan, Saskatoon,
Saskatchewan, Canada S7N 5E5.
Received 30 March 1998/Returned for modification 10 May 1998/Accepted 27 May 1998
Cells respond to heat and other forms of stress by upregulating expression of a family of highly conserved heat shock
proteins (HSPs) which function under both normal and stressful conditions as molecular chaperones mediating the folding,
assembly, translocation, and degradation of proteins (reviewed
in references 25 and 34). In eukaryotes, stress-induced expression of HSPs is regulated primarily by heat shock transcription
factor HSF1, which acts through heat shock regulatory elements (HSEs) found in the promoters of HSP genes (33, 73).
Under nonstress conditions, HSF1 exists as a repressed nonDNA-binding monomer (16, 53, 75), and the activation-deactivation pathway of HSF1 in metazoan cells involves several
independently regulated steps (reviewed in references 40 and
73). Different forms of cellular stress trigger the rapid conversion of HSF1 from inert monomers to homotrimers, and this
first step is accompanied by increased HSE-binding affinity (4,
53, 55, 66, 70). Detailed mutagenic analyses have suggested
that HSF1 monomers are stabilized by intramolecular interactions between several hydrophobic heptad repeats and that
activation during stress involves the disruption of these intramolecular interactions, leading to formation of intermolecular coiled coils with other HSF1 monomers (16, 55, 66, 75).
Thus, the suppression of HSE-binding activity under normal
conditions is regulated at least in part by hydrophobic se-
quences within HSF1, although other regions of the molecule
have also been implicated in this regulation (48). Once it has
trimerized, HSF1 undergoes additional modifications to upregulate transcriptional activity which appear to be regulated
independently of trimerization and DNA binding (21, 59, 76).
The final step of HSF1 regulation is deactivation or attenuation. Upon removal of stress, HSF1 dissociates into monomers
and ceases to activate transcription (11, 44, 52).
It has become clear that HSF1 is subject to complex regulation under both normal and stress conditions. First, a simple
model in which HSF1 is regulated by temperature alone is
contradicted by observations that hsp genes are switched on by
multiple unrelated stresses and that HSF1 molecules expressed
in heterologous systems are reprogrammed according to the
appropriate physiological temperatures of the host cells (8, 11,
30, 53). It therefore appears that HSF1 is under negative regulation by unknown cellular factors. Second, HSF1 has been
shown to be phosphorylated by several different signal transduction molecules; for example, heat-induced activation of
HSF1 was enhanced by phorbol ester treatment of human
erythroleukemia cells, and thus HSF1 is apparently targeted by
protein kinase C (26). HSF1 has also been shown to be phosphorylated by ERK1, linking HSF1 to the Ras signaling pathway (29). The constitutive phosphorylation on serine and
threonine residues and inducible hyperphosphorylation in response to stress also suggested that HSF1 is modulated by
cellular kinases and/or phosphatases (4, 30, 55). Although definitive correlations between different phosphorylation states
and specific HSF1 activities have not been established (12, 31,
* Corresponding author. Mailing address: Department of Anatomy
and Cell Biology, College of Medicine, University of Saskatchewan,
107 Wiggins Rd., Saskatoon, SK, Canada S7N 5E5. Phone: (306)
966-4069. Fax: (306) 966-4298. E-mail: ovsenekn@duke.usask.ca.
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Transcriptional activation of heat shock genes is a reversible and multistep process involving conversion of
inactive heat shock factor 1 (HSF1) monomers into heat shock element (HSE)-binding homotrimers, hyperphosphorylation, and further modifications that induce full transcriptional competence. HSF1 is controlled by
multiple regulatory mechanisms, including suppression by additional cellular factors, physical interactions
with HSP70, and integration into different cellular signaling cascades. However, the signaling mechanisms by
which cells respond to stress and control the HSF1 activation-deactivation pathway are not known. Here we
demonstrate that HSP90, a cellular chaperone known to regulate several signal transduction molecules and
transcription factors, functions in the regulation of HSF1. The existence of HSF1-HSP90 heterocomplexes was
shown by coimmunoprecipitation of HSP90 with HSF1 from unshocked and heat-shocked nuclear extracts,
recognition of HSF1-HSE complexes in vitro by using HSP90 antibodies (Abs), and recognition of HSF1 in vivo
by HSP90 Abs microinjected directly into oocyte nuclei. The functional impact of HSP90-HSF1 interactions was
analyzed by using two strategies: direct nuclear injection of HSP90 Abs and treatment of cells with geldanamycin (GA), an agent that specifically blocks the chaperoning activity of HSP90. Both HSP90 Abs and GA
delayed the disassembly of HSF1 trimers during recovery from heat shock and specifically inhibited heatinduced transcription from a chloramphenicol acetyltransferase reporter construct under control of the hsp70
promoter. HSP90 Abs activated HSE binding in the absence of heat shock, an effect that could be reversed by
subsequent injection of purified HSP90. GA did not activate HSE binding under nonshock conditions but
increased the quantity of HSE binding induced by heat shock. On the basis of these findings and the known
properties of HSP90, we propose a new regulatory model in which HSP90 participates in modulating HSF1 at
different points along the activation-deactivation pathway, influencing the interconversion between monomeric
and trimeric conformations as well as transcriptional activation. We also put forth the hypothesis that HSP90
links HSF1 to cellular signaling molecules coordinating the stress response.
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MATERIALS AND METHODS
Oocyte manipulations, stress treatments, and microinjection. Xenopus laevis
frogs were purchased from Xenopus I (Ann Arbor, Mich.). Ovary portions were
surgically removed from adult female frogs, and follicle cells were removed from
oocytes by treatment in calcium-free OR2 (45) buffer (82.5 mM NaCl, 2.5 mM
KCl, 1 mM MgCl2, 1 mM NaH2PO4, 5 mM HEPES [pH 7.8], 10 mg of streptomycin sulfate per liter, 10 mg of benzyl penicillin per liter) containing 2 mg of
collagenase (type II; Sigma) per ml for 2 to 3 h at 18°C. Oocytes were washed
extensively and allowed to recover overnight in OR2 containing 1 mM CaCl2 at
18°C, and only stage VI oocytes were selected for experiments. Nuclei were
obtained under OR2 by scoring animal hemispheres with a needle and gently
squeezing the equatorial region with watchmaker’s forceps. In some experiments,
oocytes were incubated either in OR2 containing 10 mg of GA (a gift of the
Developmental Therapeutics Program, National Cancer Institute, Bethesda,
Md.) per ml dissolved in dimethyl sulfoxide (DMSO) or in OR2 with 1:250
(vol/vol) DMSO for 2 h at 18°C, rinsed, and then exposed to heat shock treatments. In all experiments, a minimum of 20 oocytes was used for each sample.
Plasmid constructs used for microinjection experiments were the human cytomegalovirus (CMV)-chloramphenicol acetyltransferase (CAT) and the Xenopus hsp70-CAT clones (kindly provided by Alan Wolffe, National Institute of
Child Health and Human Development, National Institutes of Health, Bethesda,
Md.), which were previously described (32). Following defolliculation, oocytes
were incubated for several hours at 18°C, after which healthy oocytes were
selected and injected into nuclei with 20 nl of a solution containing 2 ng of either
CMV-CAT or hsp70-CAT plasmid per ml (40 pg) by using a Narashige IM 300
microinjector. After incubation overnight at 18°C, oocytes were subjected to heat
shock or GA treatments and then incubated for an additional 12 h at 18°C,
washed in OR2, and assayed for CAT activity.
For Ab microinjection experiments, Abs were diluted (1:1) in sterile H2O
immediately prior to microinjection. In experiments in which the effects of
HSP90 Abs on induced transcription were measured, Ab solutions (15 nl) were
injected directly into the nuclei of oocytes containing reporter constructs (injected 12 h previously). Oocytes were then incubated for 30 min at 18°C, heat
shocked, and then incubated for a further 12 h to allow for CAT expression. In
experiments to measure the effects of HSP90, HSF1, or YY1 Abs on HSF1
binding, Abs were injected into (previously uninjected) oocyte nuclei. Injected
oocytes were allowed to recover for 30 min at 18°C and then incubated at a
control temperature (18°C) or heat shocked at 33°C. Protein extracts were then
prepared for gel mobility shift analyses and immunoblotting. HSP90 polyclonal
Abs (PAbs) and purified bovine HSP90 were gifts of R. Matts, Oklahoma State
University, Stillwater; anti-HSP90 monoclonal Ab (MAb) (SPA 830) was purchased from StressGen, Victoria, British Columbia, Canada; HSF1 PAbs (55)
were a gift of K. Sarge, University of Kentucky, Lexington; and anti-Ying Yang
1 (anti-YY1) transcription factor PAbs were purchased from Santa Cruz Biotechnology, Santa Cruz, Calif.
Protein extracts and gel mobility shift assays. Protein extracts were prepared
by homogenizing oocytes in buffer C (50 mM Tris-Cl [pH 7.9], 20% glycerol, 50
mM KCl, 0.1 mM EDTA, 2 mM dithiothreitol, 10 mg of aprotinin per ml, and 10
mg of leupeptin per ml) (14) in a Dounce homogenizer (10 ml/oocyte). Homogenates were transferred to Eppendorf tubes and spun for 5 min at 15,000 3 g
(4°C). The resultant supernatants were placed in a fresh tube and then immediately frozen in liquid nitrogen and stored at 280°C.
DNA mobility shift assays were performed by using HSE oligonucleotide
probes as previously described (19). DNA-binding reaction mixtures contained
10 ml of extract (one oocyte equivalent by volume, or 20 mg of soluble protein).
The relative amounts of protein in all samples were confirmed by Bradford
assays, and extract volumes were adjusted so that equal protein concentrations
were added to each binding reaction mixture. Binding reactions were performed
with 1 mg of poly(dI-dC)–10 mM Tris (pH 7.8)–50 mM NaCl–1 mM EDTA–0.5
mM dithiothreitol–5% glycerol in a final volume of 20 ml. Reaction mixtures
were incubated on ice for 20 min and immediately loaded onto 5% nondenaturing polyacrylamide gels containing 6.7 mM Tris-Cl (pH 7.5), 1 mM EDTA, and
3.3 mM sodium acetate. Gels were electrophoresed for 2.5 h at 150 V, dried, and
exposed to autoradiography with X-ray film (XAR; Kodak). In vitro Ab recognition experiments were performed by adding Abs directly in DNA-binding
reaction mixtures or by mixing Abs with oocyte extracts for 20 min on ice prior
to DNA-binding reactions.
Immunoblotting and immunoprecipitations. Protein extracts were fractionated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE)
(10% acrylamide) and electroblotted onto polyvinylidene difluoride membranes,
and the blots were blocked (for 2 h at room temperature) in TBST (20 mM
Tris-Cl [pH 7.6], 137 mM NaCl, 0.1% [vol/vol] Tween 20) containing 5% milk
powder. Abs were diluted in TBST with 2.5% milk (1:5,000 for anti-HSP90 PAb),
and blots were incubated in primary antibody for 2 h at room temperature. Blots
were washed in TBST and incubated with secondary Ab (horseradish peroxidaseconjugated goat anti-rabbit immunoglobulin G; Bio-Rad) diluted 1:10,000 in
TBST and 2.5% milk for 2 h at room temperature. Blots were washed, and
proteins were visualized by chemiluminescence (Renaissance system; Dupont
NEN) and autoradiography with X-ray film (XAR; Kodak).
Immunoprecipitations were performed by the method described by Firestone
and Winguth (17). Nuclei from nonshocked and heat-shocked (30 min, 33°C)
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46, 72), recent evidence suggests that hyperphosphorylation
could function both to increase transactivation potential and to
delay the dissociation of trimers (74).
In addition to phosphorylation and suppression by cellular
factors, it has been hypothesized that HSPs themselves, particularly HSP70, could function as part of an autoregulatory
mechanism by which cells detect stress and regulate HSF1
(39). This idea is substantiated by a number of experimental
observations with various cell types: activated expression of
other HSPs after mutation of the hsp70 gene in yeast (9),
continued transcription of hsp70 genes during recovery in cells
blocked for HSP accumulation by treatment with translational
inhibitors (13), and activation of hsp genes in response to
artificially increased nuclear concentrations of denatured proteins (2, 37). It has been shown that physical interactions between HSP70 and HSF1 occur both in vitro and in vivo, but
these appear to be unstable, and thus the stoichiometric relationships of these complexes have not been determined (1, 3, 5,
39, 47, 53, 70). In addition, different studies have shown that
direct overexpression of HSP70 in cultured cells either reduced
the level of HSF1 activation during induction (5, 41) or increased the rate of HSF1 deactivation (41, 54), and so the precise influence of HSP70 on oligomerization steps of the HSF1
activation-deactivation have not been fully elucidated. More
recently, HSP70 has been shown to repress HSF1 through direct interaction with the transactivation domain (61); therefore, the primary autoregulatory role of HSP70 appears to be
downregulation of HSF1-mediated transcription.
Another well-characterized HSP, HSP90, functions as the
core of several chaperone heterocomplexes that contain different assortments of noncovalently linked proteins, including
HSP70 (18). Numerous studies have established that HSP90 is
required for higher-order folding of certain signal transduction
molecules and transcription factors (reviewed in references 7
and 28) and that HSP90 functions in the later stages of protein
folding after the formation of extensive secondary structure
(35, 62, 63). HSP90 is thought not to be a general chaperone
but rather to interact with and modulate the activity of various
specific substrates, including hypoxia-inducible factor a (20),
MyoD (58), steroid hormone receptors (50), the pp60v-src kinase (10), casein kinase II (15), and eIF-2a kinase (69).
Together, the known chaperone function and substrate specificity of HSP90, the conformational changes associated with
HSF1 oligomerization, apparent integration of HSF1 into different signal transduction cascades, and a previous report describing an interaction between immobilized HSP90 and HSF
from HeLa cell nuclear extracts (42) prompted us to consider
HSP90 as a potential modulator of HSF1 activity. Further,
HSF1 has recently been shown to assemble HSP90 and other
chaperones common to the progesterone receptor assembly
pathway in vitro (43). Here we investigated the potential autoregulatory role of HSP90 by using Xenopus oocytes. Results
of coimmunoprecipitation and antibody recognition experiments provide clear evidence of a physical association in vivo
between HSP90 and HSF1. Impairment of HSP90 function
either by microinjected antibodies or by the specific binding
agent geldanamycin (GA) resulted in a significant delay in
HSF1 trimer disassembly during recovery and a marked reduction in heat-induced transcription from the hsp70 promoter. In
addition, microinjected HSP90 antibodies (Abs) caused the
activation of HSE-binding activity in the absence of heat shock,
an effect which could be reversed by subsequent injection of
exogenous HSP90. Together these observations support a new
regulatory model in which formation of a heterocomplex between HSP90 and HSF1 plays a key role in modulating different steps of the HSF1 activation-deactivation pathway.
MOL. CELL. BIOL.
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REGULATION OF HSF1 BY HSP90
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FIG. 1. Identification of HSP90-HSF1 heterocomplexes in Xenopus oocyte
nuclei. (A) Immunoblot showing subcellular distribution of HSP90. Oocytes
were incubated at a nonshock (NS) temperature (18°C) or heat shocked (HS) at
33°C for 2 h, and proteins from single intact oocytes or from the nuclear and
cytoplasmic fractions were separated by SDS-PAGE along with purified bovine
HSP90 as a control. HSP90 was detected with an HSP90 PAb. The positions of
molecular mass standards are shown on the left. (B) Coimmunoprecipitation of
HSF1 and HSP90 from oocyte nuclei. HSF1 and YY1 were immunoprecipitated
with rabbit anti-mouse HSF1 PAb (55) or YY1 PAb (Santa Cruz). Nuclear
extracts were isolated from nonshocked or heat-shocked (33°C for 1 h) oocytes.
Immunoprecipitated material was subjected to SDS-PAGE and transferred to
nitrocellulose, and the blot was probed with HSP90 and HSF1 Abs as indicated.
oocytes were isolated as described above, and in each assay eight nuclei were
subjected to immunoprecipitation with anti-mouse HSF1 PAb (55) or anti-YY1
transcription factor PAb (Santa Cruz Biotechnology).
CAT assays. CAT assays were performed with one oocyte equivalent of wholecell extract from uninjected or microinjected oocytes as previously described
(49). A pool of at least 20 microinjected oocytes was used for each experimental
treatment. The acetylated products were separated by thin-layer chromatography
and visualized by autoradiography.
RESULTS
Detection of HSP90 in immune complexes with HSF1. In
this study we used the Xenopus oocyte model system to examine the potential role of the HSP90 chaperone in modulating
the activities of HSF1. We have previously shown that endogenous HSF1 is a nuclear protein in Xenopus oocytes both
before and after heat shock (36). Accordingly, we reasoned
that endogenous HSP90 must be present in oocyte nuclei in
order for HSP90-HSF1 interactions to occur in vivo. To test
this, oocytes were manually enucleated and the subcellular
distribution of HSP90 was determined by immunoblotting
(Fig. 1A). In this assay, total protein from intact oocytes was
compared directly to protein in a single large oocyte nucleus
and in the cytoplasm of the same cell. Although the majority of
HSP90 appeared to reside in the cytoplasm, HSP90 was clearly
detectable in the nuclear compartments of both unshocked and
heat shocked oocytes. Relatively equal quantities of HSP90
were detected in nuclei isolated from oocytes before and after
heat shock, demonstrating that the nucleocytoplasmic distribution of HSP90 was not significantly affected by the heat
stress conditions used in this experiment (33°C for 2 h). Heat
shock did not result in a detectable increase in the total cellular
levels of HSP90. This is in agreement with previous observations demonstrating that HSP synthesis is not significantly induced by heat shock in the single-cell Xenopus oocyte (27).
In order to determine if in vivo HSP90-HSF1 interactions
could be detected in vivo, HSF1 was immunoprecipitated from
unshocked or heat-shocked oocyte nuclei by using HSF1 PAbs
(anti-mouse HSF1 antiserum) (55), and the immunoprecipitated material was examined for the presence of HSP90 by
immunoblotting (Fig. 1B). HSP90 was present in the HSF1immunoprecipitated material from both nonshocked and heatshocked nuclei, suggesting a physical association between
HSP90 and HSF1. Identical quantities of HSP90 were detected
before and after heat shock, indicating that HSP90 associates
equally well with inactive HSF1 monomers or stress-activated
HSF1 trimers. Control experiments show that the HSF1 PAbs
used in these experiments do not cross-react with endogenous
Xenopus or purified bovine HSP90 on immunoblots (data not
shown). Also, as shown in Fig. 1B, HSP90 was not detected in
control immunoprecipitations with PAbs against YY1 (60),
which is a relatively abundant transcription factor in Xenopus
oocytes (36a).
HSP90 Abs recognize heat-activated HSF1 in vivo and in
vitro. While coimmunoprecipitation of HSP90 and HSF1 suggested a physical interaction between these proteins, additional evidence was required to substantiate the existence of
HSP90-HSF1 heterocomplexes. In the following series of experiments, we tested the ability of HSP90 Abs to recognize
activated DNA-binding HSF1 trimers. HSP90 Abs were mixed
with aliquots of unshocked or heat-shocked oocyte extracts
prior to DNA-binding reactions with radiolabeled HSE probes,
and the effect on the migration of HSF1-HSE complexes was
analyzed in gel mobility supershift assays (Fig. 2). Heat-induced HSF1 complexes were clearly supershifted by an HSP90
MAb and were disrupted by HSP90 PAbs, although in comparison to the HSP90 MAb, a relatively large amount of antiserum was required for this effect. The results of these exper-
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FIG. 2. Recognition of the heat shock-activated DNA-binding form of HSF1
in gel mobility shift assays by HSP90 Abs. Aliquots of nonshocked (NS) or
heat-shocked (HS) oocyte extracts were mixed with HSP90 MAb or PAbs (antiserum) and incubated for 30 min prior to DNA-binding reactions with a 32Plabeled HSE probe, and the migration of HSF1-HSE complexes was analyzed in
gel mobility shift assays. Lanes in which extracts were not mixed with antibodies
are indicated (2), and the final antibody dilutions in the extract incubations are
indicated above the panel. Positions of the nonspecific binding activity (ns) and
heat-induced (HSF1) and supershifted (HSF1 1 ab) complexes are indicated.
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MOL. CELL. BIOL.
iments were identical for each antibody when HSP90 MAbs or
PAbs were mixed directly into DNA-binding reaction mixtures
without preincubation (data not shown). We attributed the
retardation or disruption of heat-induced HSE bandshifts observed in these assays to specific antibody recognition of
HSP90 in direct association with HSF1 trimers. This interpretation is consistent with the coimmunoprecipitation of HSP90
and HSF1 (Fig. 1). The disruption of HSE binding by HSP90
PAbs, while differing somewhat from the clear supershift by
the MAb, was similar to the results of supershift experiments
with HSF1 PAbs (see Fig. 4B) (19). It is likely that recognition
of multiple epitopes by PAbs present in HSP90 antiserum
interfered with HSP90-dependent DNA-binding activities of
HSF1.
We next determined if the physical association between
HSP90 and HSF1 implied by in vitro supershift experiments
could be detected in vivo. A series of experiments in which
HSP90 was targeted by direct introduction of HSP90 Abs into
oocyte nuclei was performed. It was important, however, to
first evaluate the efficacy of the Ab injection technique. Initial
experiments were aimed at determining the stability of injected
Abs during heat shock, as well as any quantitative effects that
Ab injection might have on endogenous HSP90 levels. Follow-
ing nuclear microinjection of HSP90 MAb, oocytes were allowed to recover for 1 h at 18°C and then were further incubated under nonshock or heat shock conditions (18 or 33°C for
2 h). Cell homogenates were then examined for the presence of
anti-HSP90 MAbs by immunoblotting; in this experiment injected Abs were recognized by the secondary Ab used to detect
endogenous HSP90 (Fig. 3A). The results show that both the
quantity and apparent molecular sizes of microinjected MAbs
were similar before and after heat shock and also that HSP90
levels were not affected by the presence of MAbs. Thus, it
appeared that this technique could effectively be used to target
or sequester HSP90 into immune complexes in vivo without
affecting total HSP90 concentrations.
The effect of microinjected HSP90 Abs on the DNA-binding
activity of HSF1 was analyzed both before and after heat
shock. HSP90 MAbs significantly retarded the mobility of heat
shock-induced HSF1-HSE complexes (Fig. 3B). The relative
amounts of induced HSF1 were similar in both uninjected and
HSP90 MAb-injected oocytes, indicating that these antibodies
did not suppress trimerization or the DNA-binding activity of
HSF1. Supershift of heat shock-induced HSF1-HSE complexes
was not observed after injection of HSP90 PAbs (Fig. 3C), but
this was not surprising, since the maximum volume of anti-
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FIG. 3. HSP90 interacts with HSF1 in vivo. (A) Immunoblot of nonshocked (NS) and heat-shocked (33°C, 2 h) oocytes that were either uninjected or microinjected
with HSP90 MAb. HSP90 was detected with HSP90 primary Ab, and injected HSP90 Ab was detected with the secondary antibody (as described in Materials and
Methods). Endogenous oocyte HSP90 and injected HSP90 MAbs present in extracts are indicated on the right. The positions of molecular mass standards are indicated
on the left. (B) The migration of HSF1-HSE complexes was analyzed by gel mobility shift assay of uninjected or HSP90 MAb-injected oocytes that were not shocked
(NS) or heat shocked (HS) at 33°C for the times indicated. The heat-activated HSF-HSE (HSF1) and HSP90 Ab-supershifted (HSF11ab) complexes are indicated on
the right. ns, nonspecific binding activity. (C) An experiment similar to that for panel B was performed with HSP90 antiserum (pAb).
VOL. 18, 1998
REGULATION OF HSF1 BY HSP90
4953
serum that could be injected into oocyte nuclei was much less
than the amount required to disrupt HSF1-HSE bandshifts in
vitro (Fig. 2). Despite the differences between the effects of
HSP90 MAb and PAbs seen in these experiments, the overall
results of Ab injections are consistent with our interpretations
of immunoprecipitation and in vitro gel supershift experiments
and provide further evidence that HSP90 associates with HSF1
in vivo.
HSP90 associates with HSF1 in vivo. In control experiments
we also tested the effects of microinjected HSF1 Abs on HSF1
in vivo. We have previously shown that anti-mouse HSF1 PAbs
(55) supershift or obliterate stress-activated HSF1 complexes
in oocyte extracts in vitro (19). Similar to the results of in vitro
gel supershifts, direct nuclear injection of HSF1 antiserum
appeared to completely inhibit the formation of HSF1-HSE
complexes in response to heat shock (Fig. 4B). We could not,
however, distinguish between the possibility that HSF1 Abs
actually bound to and inhibited the trimerization of inactive
HSF1 monomers prior to heat shock and the possibility that
HSF1 Ab-supershifted complexes were simply not detectable
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FIG. 4. HSP90 Abs activate the HSE-binding activity of HSF1 under nonstress conditions. (A) Left, gel mobility shift assay of uninjected oocytes (2) and HSP90
Ab-injected oocytes (MAb or PAb) that were incubated at nonshock temperatures (NS) or heat shocked for 1 h (HS). The positions of heat-activated or Ab-activated
HSF1-HSE complexes (HSF1) and HSP90 Ab-supershifted complexes (HSF1 1 ab) are indicated. Right, gel mobility shift assay showing the effects of coinjected bovine
HSP90 on the formation of HSF1-HSE complexes. In the third lane (1HSP90), 50 ng of purified bovine HSP90 was injected into oocytes 2 h after injection of HSP90
PAbs, and extracts were made following incubation at a nonshock temperature for a further 1 h. ns, nonspecific binding. (B) Bottom, gel mobility shift assay of oocytes
injected with PAbs against YY1 (Santa Cruz; see Materials and Methods). Top, comparison of the activation of HSF1 in uninjected (uninj) and sham-injected (H2O)
or HSF1 antiserum-injected (HSF1 PAb) (55) oocytes. (C) Recognition of HSF1 by microinjected HSP90 Abs occurred in vivo. A gel mobility shift assay was performed
with uninjected (2) or HSP90 MAb-injected oocytes. In some lanes, HSP90 MAb or purified bovine HSP90 was added to extracts as indicated below the panel. HSP90
(1 mg) was added to Ab-injected oocytes at the time of homogenization (fifth lane from left), and HSP90 MAb or protein (1 mg) was added to uninjected heat-shocked
oocyte extracts just prior to DNA-binding reactions (seventh and eighth lanes from left).
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MOL. CELL. BIOL.
FIG. 5. HSP90 Abs delay the deactivation of HSF1 in vivo. Uninjected or HSP90 MAb-injected oocytes were subjected to a heat shock at 33°C for 1 h (HS) and
then allowed to recover at 18°C for the periods of time indicated, and the HSE-binding activity of HSF1 was analyzed by gel mobility shift assay. Both heat-induced
and supershifted complexes are indicated as HSF1 at the left. ns, nonspecific binding activity.
Nuclear injection of HSP90 Abs delays attenuation of HSF1binding activity. Taken together, the established role of HSP90
as a protein-folding chaperone targeting specific cellular substrates, the known structural changes associated with HSF1
oligomerization, and the results here of immunoprecipitation
and antibody recognition experiments demonstrating association between HSP90 and HSF1 and activation of HSF1 by
microinjected HSP90 Abs suggested that HSP90 could function as a modulator of HSF1 activity. To examine this further,
we analyzed the effects of microinjected HSP90 Abs on the
DNA-binding properties of HSF1 during the deactivation
phase of the heat shock response. Since HSF1-HSE complexes
persist indefinitely throughout prolonged heat shock treatments of oocytes (19), we could not monitor HSF1 deactivation during continuous stress. Instead, we compared attenuation of HSE binding in uninjected and HSP90 Ab-injected
oocytes recovering at 18°C from a 33°C heat shock. As shown
in Fig. 5, microinjected HSP90 MAbs caused a significant delay
in the attenuation of HSE-binding activity relative to that for
uninjected controls. The result of this experiment suggested
that disruption of the chaperoning ability of HSP90 in vivo
decreased the rate of HSF1 trimer disassembly. Thus, it appears that, in addition to suppressing trimerization, HSP90
also plays a role in the conformational changes required for the
reverse step of HSF1 trimerization.
HSP90 Abs inhibit HSF1-mediated transcription from hsp70
promoter. The transcriptional activation domain of HSF1 is
regulated independently of trimerization, and it has been hypothesized that full induction requires additional conformational changes (21, 59, 76). We postulated that HSP90-HSF1
interactions might also have a functional impact on heat-inducible transcription. To investigate this possibility, we measured the effect of HSP90 Abs on heat-inducible expression
from hsp70-CAT reporter constructs. Nuclear injection of
HSP90 Abs resulted in a significant reduction of heat-induced
CAT activity (Fig. 6A). The negative effect on HSF1-mediated
CAT expression implied that binding of Abs to HSP90 in vivo
could disrupt transactivation by HSF1 under heat shock conditions. In order to rule out the possibility that injected HSP90
Abs had a general inhibitory effect on transcription and/or
CAT, parallel experiments were performed with CMV-CAT
reporter constructs (Fig. 6A). The results show that expression
from the CMV promoter was not affected by anti-HSP90. In
additional control experiments, we assessed the effects of
HSF1 Abs on heat shock-induced expression of hsp70-CAT.
The results show that CAT activity was downregulated by
HSF1 antiserum (Fig. 6B), and, as was the case for HSP90
MAbs, expression from the CMV promoter was not affected by
HSF1 antiserum. Thus, the negative effects of both HSP90 and
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in these gel shift assays. The results of control experiments also
showed that heat shock-induced HSF1-HSE complexes were
not affected by sham nuclear injection of H2O or nuclear
injection of Abs against the transcription factor YY1 (Fig. 4B),
indicating that the supershift of heat-induced HSF1-HSE complexes by microinjected HSP90 Abs was attributable to specific
effects on HSP90 in vivo.
HSP90 Abs induce HSE binding in the absence of heat
shock. In the mobility gel shift assay with the results shown in
Fig. 3B, HSF1-HSE complexes were not observed in unshocked oocytes after nuclear injection of an HSP90 MAb;
however, in repeated experiments HSF1 appeared to be induced by the same MAb in the absence of heat shock (Fig. 4A).
Similar heat shock-independent activation was also observed in
experiments with microinjected PAbs in HSP90 antiserum
(Fig. 3C and 4A). Thus, it appeared that injection of HSP90
Abs mimicked the effects of heat shock in eliciting the DNAbinding activity of HSF1. The mobility of HSF1-HSE complexes induced by injected HSP90 Abs in unshocked oocytes
did not appear to be supershifted but was similar to that of the
heat-induced complexes in uninjected cells (Fig. 3C and 4A).
This suggested that Ab-induced activation may have been the
result of Ab interaction with HSP90 not in direct contact with
HSF1.
Several control experiments were performed to determine if
the activation of HSF1 under nonshock conditions by microinjected HSP90 Abs was attributable to specific effects on
HSP90 in vivo. First, induction of HSE binding by HSP90 Abs
could be reversed by subsequent injection of 50 ng of purified
HSP90 (Fig. 4A). Second, sham injection or the nuclear presence of YY1 or HSF1 Abs did not induce HSE binding in the
absence of heat shock (Fig. 4B). Thus, HSF1 activation by
HSP90 Abs was not due to stress caused by the injection
procedure and/or the presence of nonspecific antibodies alone
but rather was a result of specific Ab interactions with HSP90
in vivo. Since injected HSP90 Abs probably inhibit or disrupt
HSP90 function in vivo, these results indicate that HSP90 participates in repression of trimerization, or maintaining the monomeric state of HSF1, under nonshock conditions. We also
found that gel supershifts of activated HSF1-HSE complexes
by HSP90 MAb added in vitro could be quenched or inhibited
by the addition of excess purified HSP90 (1 mg/oocyte) to
DNA-binding reaction mixtures (Fig. 4C). Addition of the
same quantity of excess HSP90 to heat-shocked oocytes immediately upon homogenization did not suppress the supershift of
HSF1-HSE complexes by injected HSP90 MAb (Fig. 4C). We
therefore conclude that the effects on HSF1-HSE complexes
observed after microinjection of HSP90 MAbs were the result
of Ab interactions occurring in vivo.
VOL. 18, 1998
REGULATION OF HSF1 BY HSP90
4955
HSF1 Abs appeared to be specific to HSF1. On the basis of
these expression assays, we suggest that HSP90 Ab interactions
with HSF1-HSP90 heterocomplexes in vivo hindered or inhibited physical changes to HSF1 required for the acquisition of
full transcriptional competence in response to heat shock. We
have not ruled out the possibility that the inhibitory effect of
injected Abs was exerted through interaction with uncomplexed HSP90 molecules. It is also possible that HSP90 must
dissociate from HSF1 trimers before transcriptional activation,
and therefore Abs may have sterically inhibited conformational changes to the HSF1 activation domain.
Effect of GA on the DNA-binding and transcriptional activities of HSF1. In order to rule out potential artifacts of microinjected Abs, we examined the role of HSP90 in regulating
HSF1 activities by an alternative method involving treatment
of cells with GA. GA is a fungal benzoquinoid ansamycin
known to block HSP90 function (65, 71). GA binds specifically
to HSP90 at the ATP-binding domain (22, 51, 67) and prevents
HSP90-mediated renaturation of proteins (24, 56, 68). GA has
been shown to inhibit the HSP90-dependent activities of numerous proteins, including the glucocorticoid receptor and
pp60v-src (71), the Src family kinase p56lck (23), Raf kinase
(57), and progesterone receptor (65).
Oocytes were incubated in medium supplemented with GA
(10 mg/ml) for 2 h, and the DNA-binding activity of HSF1 was
analyzed by gel shift assay with labeled HSE. As shown in Fig.
7A, GA treatment alone did not activate HSF1. We did not
observe HSF1 induction by GA at nonshock temperatures
even after extended exposure of up to several hours or after
acute exposure to GA concentrations as high as 100 mg/ml
(data not shown). The failure of GA to induce HSE binding in
the absence of heat shock indicated that GA inhibition of
HSP90 in vivo was not sufficient to bring about HSF1 trimerization. In heat shock experiments, however, GA treatment (10
mg/ml) for 2 h prior to a heat shock of 30°C for 30 min caused
a significant increase in the induced levels of HSF1-HSE complexes relative to those for identically stressed controls (Fig.
7A). Elevated levels of HSE-binding activity were observed at
several different points during the induction phase, from between 5 min and 1 h of heat shock.
Enhanced activation of HSF1 in GA-treated oocytes could
have been attributed to a synergistic effect of heat shock in
combination with GA or to a shift in the oligomerization equilibrium towards formation of HSF1 trimers caused by GA
inhibition of HSP90-mediated dissociation. In an attempt to
distinguish between these possibilities, we examined the effect
of GA on the DNA-binding properties of HSF1 during the
deactivation phase of the heat shock response. The results of
recovery experiments consistently showed that deactivation of
HSF1 was delayed in GA-treated oocytes relative to that in
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FIG. 6. Microinjected HSP90 Abs inhibit the heat-induced transcriptional activity of HSF1. (A) CAT assay of oocytes microinjected with hsp70-CAT (left) or
CMV-CAT (right). Oocytes were injected with HSP90 MAb 1 h before heat shock (33°C for 1 h) treatment (HS), and the expression from each promoter was compared
directly to that in similarly treated oocytes without injected Ab. Each experiment was repeated at least five times with different batches of oocytes, yielding similar
results. The positions of unacetylated (Cm) and acetylated (Ac) forms of [14C,]chloramphenicol are indicated on the left of each panel. Background CAT activity of
non-plasmid-injected oocytes is shown in the right panel (cont). NS, no heat shock. (B) CAT assay of oocytes microinjected with reporters as described above. HSF1
PAbs were injected into oocyte nuclei essentially as described above for HSP90 Ab.
4956
ALI ET AL.
MOL. CELL. BIOL.
untreated controls (Fig. 7B). It is important to note that even
after severe heat treatments, HSE binding of endogenous
HSF1 in oocytes returns to control levels within 5 to 15 min of
recovery (19). Thus, GA disruption of the chaperoning activity
of HSP90 in vivo appears to decrease the rate of HSF1 trimer
disassembly. The results of recovery experiments with GA
were similar to those of HSP90 Ab injections, and together
these data imply that HSP90 functions in controlling the conformational changes associated with conversion of HSF1 from
trimers to monomers. From these experiments, however, we
cannot be certain whether GA and HSP90 Abs exerted their
effects directly by inhibiting the function of HSP90 molecules
in complex with HSF1 or indirectly by inhibiting the function
of HSP90 molecules not stably complexed with HSF1.
We next used GA to examine the functional significance of
HSP90-HSF1 interactions for heat-inducible transcription. Expression from microinjected hsp70-CAT reporter constructs in
untreated and GA-treated oocytes under nonshock and heat
shock conditions was compared (Fig. 8). The results show that
heat-induced CAT activity was significantly reduced in GAtreated oocytes relative to that in untreated controls. Thus,
inhibition of HSF1-mediated CAT expression by GA mimicked the results with microinjected HSP90 Abs. Expression
was unaffected by GA in parallel experiments with microinjected CMV-CAT reporter constructs (Fig. 8), thus ruling out
the possibility that GA had a general inhibitory effect on transcription and/or CAT activity in oocytes. The results of expression experiments using both microinjected Abs and GA strongly suggest that HSP90 functions in regulating conformational
changes to the transcriptional activation domain of HSF1.
DISCUSSION
HSF1 is one of the most intensely studied eukaryotic transcription factors. Numerous reports have described the functional domains and intrinsic properties of HSF1 molecules
from different organisms, the changes in oligomerization and
phosphorylation states, interactions with HSP70, and HSF1
activation by disparate stress signals. Despite knowledge of
these events, however, the precise mechanisms by which cells
regulate HSF1 are yet to be defined. This study is an important
step towards understanding HSF1 regulation. It had previously
been postulated, on the basis of HSP90-HSF1 heterocomplex
FIG. 8. GA inhibits the heat-induced transcriptional activity of HSF1. CAT
activities of CMV-CAT- or hsp70-CAT-injected oocytes that were either untreated or treated with GA (10 mg/ml, 2 h) prior to incubation at 18°C (NS) or
heat shock (HS) (33°C for 2 h) were compared. Each experiment was repeated
at least five times with different batches of oocytes, yielding similar results. The
positions of unacetylated (Cm) and acetylated (Ac) forms of [14C]chloramphenicol are indicated on the left.
Downloaded from http://mcb.asm.org/ on February 2, 2016 by guest
FIG. 7. (GA) enhances the heat-inducible HSE-binding activity of HSF1 and delays deactivation during recovery. (A) Oocytes were incubated for 2 h in the presence
of 10 mg of GA (G) per ml or in DMSO (D) or left untreated (U) prior to heat shock at 30°C for the times indicated. The HSE-binding activities of HSF1 were then
compared by gel mobility shift assay. (B) Gel mobility shift assay of oocytes that were treated with GA or DMSO as described above, heat shocked at 33°C for 1 h and
then allowed to recover at 18°C for the periods of time indicated. The positions of activated HSF1 and nonspecific (ns) bands are indicated at the left of each panel.
VOL. 18, 1998
4957
FIG. 9. Model of HSF1 regulation by HSP90. Our data indicate that HSP90
interacts with the inactive monomeric and active trimeric forms of HSF1 and
could be involved in regulating the interconversion between these forms (A and
C). Both HSP90 Abs and GA delay trimer disassembly and inhibit heat-induced
transcription of HSF1 (D). These observations suggest a role for HSP90 in
modulation of the transcriptional activation domain (B) or that HSP90 dissociation from trimers is required for transcriptional competence. We also hypothesize that HSP90 could link HSF1 to cellular signaling molecules (B). Details of
HSF1 structure are not shown, although the shape change from a circle to an
ellipse is intended to indicate conformational changes associated with trimerization. No data are presented for the one-to-one stoichiometric relationship between HSP90 and HSF1, which is shown only for simplicity. Although HSP70 is
not represented here, we do not imply its absence of involvement in any of these
processes.
model shown in Fig. 9 describes a hypothetical mechanism by
which HSF1 could be linked to disparate signaling pathways.
We put forth the hypothesis that HSP90 functions as an integral part of the cellular stress detection machinery, tethering
HSF1 to cell signaling molecules. We suggest that this could
occur through transient or reiterative interactions between
HSP90-HSF1 heterocomplexes and other HSP90-associated
cellular kinases or other regulatory molecules. Alternatively,
HSF1 could assemble with HSP90 and other molecules into
large heteromeric complexes; however, to our knowledge these
have not been detected in vivo through biochemical approaches. Our proposal of interactions between HSP90 molecules simultaneously in complex with HSF1 and cellular signaling molecules provides a plausible mechanism by which
signaling molecules that participate in regulating the cellular
stress activation-deactivation pathway could be specifically presented to HSF1. The formation of HSP90 homodimers (38) is
consistent with this idea.
Specific recognition of HSF1-HSE complexes by using
HSP90 Abs was demonstrated both in vivo (Abs injected into
oocyte nuclei) and in vitro (Abs added to DNA-binding reactions mixtures). Also, both active and inactive forms of HSF1
coimmunoprecipitated with HSP90 from heat-shocked and unshocked nuclear extracts. These results are consistent with the
previous findings showing an interaction between immobilized
HSP90 and HSF in nuclear extracts of unshocked HeLa cells
(42) and between HSP90 and bacterially expressed HSF1 in a
cell-free reticulocyte lysate assembly system (43), but they are
in contrast to results of other studies in which HSF1-HSP90
interactions were not detected by immunoprecipitation (53) or
supershift analyses (5); it is possible that such discrepancies
could be accounted for by the use of different Abs. The simplest interpretation of our results is that a subset of nuclear
HSP90 molecules exist in heterocomplexes with inactive HSF1
monomers and remain associated with trimers. Coimmunoprecipitation of equal quantities of HSP90 under both nonshock
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formation in vitro, that HSP90 could modulate HSF1 activity
(42, 43), an idea that has not been confirmed before this work.
The results of this analysis, using novel experimental approaches made possible by the Xenopus oocyte model system,
clearly establish a physical association between HSP90 and
endogenous HSF1 in vivo and provide evidence that HSP90
functions to modulate oligomerization and possibly the inducible transcriptional activity of HSF1.
The existence of HSP90-HSF1 heterocomplexes is corroborated by several independent lines of evidence, including the
detection of HSP90 in HSF1 Ab-immunoprecipitated material
from both heat-shocked and control cells, HSP90 Ab-directed
induction of HSE-binding activity, HSP90 Ab-directed supershifts of heat-activated HSF1 both in vitro and in vivo, and the
delayed deactivation of HSF1-binding activity after disruption
of HSP90 with GA or microinjected Abs. Results showing
HSP90 Ab induction of HSF1 in the absence of heat shock and
delayed deactivation of HSF1 after targeting of HSP90 in vivo
suggest that HSP90 contributes to the conformational changes
required for suppression of trimerization as well as disassembly
of HSF1 trimers. Furthermore, independent experiments with
microinjected HSP90 Abs and cell treatments with GA show
specific effects on heat-induced transcriptional activity of
HSF1. Although we have not directly confirmed the universality of these findings, we assume that HSP90-HSF1 interactions
and the apparent regulatory role of HSP90 are not unique to
Xenopus oocytes; in fact, the likelihood that HSP90-HSF complexes must exist in other cells is strengthened by the evolutionary conservation of these molecules and previous reports
of HSP90-HSF interactions in human cell extracts or reticulocyte lysates (42, 43). Also of note is the recent observation by
Farkas et al. (16) of a 94-kDa protein species copurifying with
bacterially expressed HSF1; our results lead us to suggest that
this could have been HSP90.
Based on the evidence presented here and the previously
reported properties of both HSF1 and HSP90 molecules, we
propose a regulatory model in which HSP90 functions as an
HSF1-specific chaperone at different points along the activation-deactivation pathway: maintenance of the monomeric
conformation under nonstress conditions, induction of transactivation potential, and disassembly of trimers to a non-DNAbinding state following stress (Fig. 9). The model predicts that
HSP90 is associated with HSF1 at different points along the
activation-deactivation pathway and exerts its regulatory effects directly through interactions with HSF1. The model also
predicts that, in addition to coordinating the dynamic changes
in HSF1 oligomerization, HSP90 could function in the unmasking of the transcriptional activation domain. Previous
studies have shown that the carboxy-terminal transcriptional
activator domain is regulated independently of oligomerization
(21, 31, 46, 59, 76). The effects on HSF1-dependent transcription of GA and microinjected HSP90 Abs suggest that this step
could be regulated at least in part by complexed HSP90. It
must be emphasized that the current model does not exclude
the involvement of HSP70 in controlling HSF1 activities.
Rather, it is more likely that HSP70 and HSP90 act coordinately in this regard. The idea of cooperative and/or coordinate regulation of HSF1 by both HSPs is congruent with the
previous identification of HSP70 and HSP90 in large chaperone complexes (63, 64), with the ability of HSF1 to assemble
HSP90 along with HSP70 and several components of the cytoplasmic molecular chaperone machinery, and with the recent
demonstration that HSP70 binds to the activation domain and
negatively regulates transcription (61).
Apart from providing a mechanistic framework in which the
HSF1-regulatory function of HSP90 could be explained, the
REGULATION OF HSF1 BY HSP90
4958
ALI ET AL.
function would be entirely consistent with the known chaperoning role of HSP90 with other cellular proteins and transcription factors such as MyoD (58). Similar to the case for the
delay of deactivation by HSP90 antibodies or GA, we have not
yet determined whether these agents inhibited transcription
directly through perturbing the function of HSP90 molecules
complexed with HSF1 or indirectly through interactions with
the uncomplexed pool of HSP90. Furthermore, additional factors such as HSP70 (61) or various signal transduction molecules could be involved in regulating the transcriptional activation domain.
There were two apparent contradictions in the results of
expression experiments: (i) under heat shock conditions, GA
treatment enhanced the HSE-binding activity of HSF1 while
reducing transcriptional activity, and (ii) under nonshock conditions, HSP90 Ab injection induced HSE binding but not a
corresponding increase in HSF1-mediated transcription from
the HSP70 promoter. It could be argued that these results
reflect the independent regulation of (i) trimerization and
DNA binding and (ii) transactivation (21, 59, 76). In fact, the
DNA-binding and transcriptional activities of HSF1 are also
uncoupled in Xenopus oocytes (6).
Rather than a direct role for HSP90 in regulating transcriptional competence of HSF1, the observations of repression by
GA and HSP90 Abs could indicate that dissociation of HSP90
from trimers is a necessary precondition for transcriptional
activation. In this case, reduced transcriptional activity caused
by HSP90 Abs or GA could have been the result of inhibition
of HSP90 release and steric blocking of modifications to the
transcriptional activation domain. It is possible that in our
experiments with oocytes, these agents acted to stabilize a
transcriptionally incompetent intermediate in the activation
pathway. Regardless of the interpretations of these experiments, however, it is clear that HSP90 is involved in the transcriptional activation of HSF1. The precise details of this role
will probably require more knowledge of the stoichiometric
relationships between these two proteins throughout the activation-deactivation pathway.
In conclusion, the data presented here provide the first clear
evidence in support of a role for HSP90 in the regulation of
HSF1. This study raises a number of key questions. For example, what is the molecular basis for HSP90-HSF1 interactions,
and what are the dynamics of HSP90-HSF1 interactions
throughout the stress response? Do ATP hydrolysis and HSF1
hyperphosphorylation affect the physical association between
these molecules, or does HSP90 actually coordinate HSF1
phosphorylation through interactions with other factors? Do
HSP70 and HSP90 cooperate in modulating HSF1 activities,
and are other molecules involved? It will be interesting to
unravel the precise roles of HSP90 in coordinating the physical
changes associated with induction of HSF1 by stress and to
determine if HSP90 plays a role in the signaling pathways used
to sense stress and coordinate the appropriate changes in cellular activities.
ACKNOWLEDGMENTS
This work was supported by a research grant from the Medical
Research Council of Canada to N.O., a postdoctoral fellowship from
the Health Services Utilization Research Council of Saskatchewan to
A.A., and a PGSA scholarship from the Natural Sciences and Engineering Research Council of Canada to S.B.
We thank S. Hartson and R. Matts for purified HSP90 protein and
antiserum; A. Wolffe for hsp70-CAT and CMV-CAT constructs; K.
Sarge and R. Morimoto for HSF1 antiserum; A. Hnatov, J. Davies, and
J. Xavier for technical assistance; and P. Mercier for critical reading of
the manuscript.
A. Ali and S. Bharadwaj contributed equally to this paper.
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and heat shock conditions implies that HSP90-HSF1 heterocomplex formation is independent of the oligomeric state of
HSF1. What remains to be determined is the relative affinity
between HSP90 and HSF1 monomers and trimers and the
stoichiometric relationship between these molecules. Also of
interest is whether HSP90 remains associated with HSF1
throughout activation and deactivation or whether HSP90 interacts reiteratively and transiently with HSF1 at each step in
the pathway.
Induction of the HSE-binding activities of HSF1 under nonshock conditions after nuclear injection of HSP90 Abs, although not after GA treatment, leads us to hypothesize that
HSP90 participates as a molecular chaperone in the folding of
HSF1 monomers, either to establish the formation of intramolecular hydrophobic interactions of inactive HSF1 monomers,
or to maintain the monomeric conformation. Interestingly, the
HSP90 Ab-induced HSE-binding activity in unshocked oocytes
did not appear to be supershifted, suggesting that this heat
shock-independent activation could have been the result of Ab
interaction with uncomplexed HSP90. Why did HSP90 Abs but
not GA induce HSF1 in unstressed cells? Presumably, the
binding of Abs to certain epitopes was sufficient to inhibit the
apparent HSP90-mediated repression of HSF1, either directly
through interactions with molecules in HSF1-HSP90 heterocomplexes or indirectly through interactions with HSP90 not
complexed with HSF1. The fact that GA treatments failed to
activate HSF1 does not necessarily contradict a potential role
for HSP90 in the folding or repression of HSF1 monomers.
GA binds the ATP-binding domain of HSP90 and is hypothesized to inhibit substrate release or lead to substrate degradation (22, 24, 51, 56, 67, 68), effects that would not inexorably
lead to trimerization.
Although the functional implications of an interaction between HSP90 and inactive monomers is still unclear, recovery
experiments indicated a clear relationship between HSP90 and
the disassembly of HSF1 trimers. Both GA treatments and
nuclear injection of HSP90 Abs significantly delayed deactivation of HSF1 during recovery from heat shock. A number of
potential mechanisms by which HSP90 might modulate reversion of HSF1 oligomerization could be considered. One is that
HSP90 acts to stabilize the trimeric conformation of HSF1 and
must be released prior to conversion to the monomeric form
and deactivation of HSE binding. Alternatively, HSP90 could
remain complexed with HSF1 throughout the disassembly process and act to mediate the conformational and folding
changes associated with conversion of homotrimers to monomers. In either case, the delay in HSF1 deactivation by GA is
probably due to inhibition of the ATPase activity of HSP90,
but the mechanism by which HSP90 Abs inhibited HSP90
function in vivo is not known. It was interesting that HSP90
Ab-induced HSF1-HSE complexes detected in gel mobility
shift assays were not supershifted. Consequently, we have not
ruled out the possibility that GA or HSP90 Abs could have
exerted their effects indirectly by affecting the chaperone activity of HSP90 molecules not in complex with HSF1.
Another key finding of this study is the repression of heatinducible transcription after targeting HSP90 in vivo. It is very
unlikely that this transcriptional repression of HSF1 was artifactual, since it was reproduced by two independent methods,
microinjection of HSP90 Abs and cell treatments with GA.
The results imply that physical binding of HSP90 by injected
Abs or disruption of its ATPase domain by GA somehow
blocked the conformational changes to the transactivation domain of HSF1. Therefore, we suggest, but have yet not confirmed, that the chaperoning function of HSP90 may be directly involved in this step of the activation pathway. This
MOL. CELL. BIOL.
VOL. 18, 1998
REGULATION OF HSF1 BY HSP90
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