Too much or too little is not good
The role of heat shock proteins (HSPs) in protein folding,
transport and complex formation has been studied extensively
(Bukau and Horwich, 1998; Hartl, 1996). Their roles in signal
transduction have been established from observations that
Hsp90 and Hsp70 are associated with a number of signaling
molecules, including v-Src, Raf1, Akt and steroid receptors
(Dittmar et al., 1998; Sato et al., 2000; Xu and Lindquist,
1993). Decreasing the levels of functional Hsp90 in Drosophila
by genetic mutation or by treatment with an Hsp90 inhibitor
geldanamycin causes developmental abnormalities (Rutherford
and Lindquist, 1998). Likewise, increasing the levels of Hsp70,
by gene transfer mediated overexpression or by heat shock, has
growth-inhibitory effects on mammalian tissue culture cells
and in Drosophila salivary gland cells, whereas expression of
a dominant-negative form of Hsp70 causes developmental
defects in Drosophila (Elefant and Palter, 1999; Feder et al.,
1992). However, what remains less well understood is whether
this is a general strategy used by the cell to link specific
signaling pathways with cell-stress-sensing events.
Interestingly, cells that have lost their ability to regulate cell
growth, such as tumor cells, often express high levels of
multiple HSPs compared with their normal parental cells
(Jaattela, 1999). Depletion of Hsp90 by geldanamycin or of
Hsp70 by anti-sense methodology in transformed cells, but not
in their non-transformed counterparts, causes either arrest of
cell growth or cell death (Nylandsted et al., 2000; Whitesell et
al., 1994). Tumor cells appear to be dependent on increased
levels of HSPs, although why this is beneficial has yet to be
clearly established. One possibility is the ability of chaperones
to suppress and buffer mutations that accumulate during the
transformation process, which could promote cell viability and
even enhanced cell growth of otherwise mutant cells. This is
exemplified by the relationship between p53, Hsp70 and
Hsp90, where mutant forms of p53, but not wild-type p53,
depend on Hsp70 and Hsp90 for normal level and function
(Blagosklonny et al., 1996; Pinhasi-Kimhi et al., 1986).
Hsp70 and Hsp90 are heat shock proteins
When cells are exposed to conditions of proteotoxic stress - for
example heat shock - the expression of HSPs, including
members of the Hsp100, Hsp90, Hsp70, Hsp60, Hsp40 and
small HSP families, is induced (Morimoto, 1998; Parsell and
Lindquist, 1993). HSPs have a critical role in the recovery of
cells from stress and in cytoprotection, guarding cells from
subsequent insults. They protect stressed cells by their ability
to recognize nascent polypeptides, unstructured regions of
proteins and exposed hydrophobic stretches of amino acids.
In doing so, chaperones hold, translocate or refold stress-
denatured proteins and prevent their irreversible aggregation
with other proteins in the cell (Bukau and Horwich, 1998;
Morimoto, 1998; Parsell and Lindquist, 1993).
In addition to their roles in protecting cells from stress,
nearly all HSPs are constitutively expressed under normal
growth conditions, where they function to maintain protein
homeostasis by regulating protein folding quality control. The
chaperone activities of heat shock proteins enable folding of
newly synthesized proteins and assist protein translocation
across intracellular membranes (Hartl and Hayer-Hartl, 2002;
Here, we focus on the activities of two abundantly expressed
and highly conserved heat shock proteins: Hsp70 and Hsp90.
The levels of human Hsp70 and Hsp90 vary among primary
Heat shock proteins interact with multiple key components
of signaling pathways that regulate growth and
development. The molecular relationships between heat
shock proteins, various signaling proteins and partner
proteins appear to be critical for the normal function of
signal transduction pathways. The relative levels of these
proteins may be important, as too little or too much
Hsp70 or Hsp90 can result in aberrant growth control,
developmental malformations and cell death. Although the
functions of heat shock proteins as molecular chaperones
have been well characterized, their complementary role as
a ‘stress-induced’ proteins to monitor changes and alter the
biochemical environment of the cell remains elusive.
Genetic and molecular interactions between heat shock
proteins, their co-chaperones and components of signaling
pathways suggest that crosstalk between these proteins can
regulate proliferation and development by preventing or
enhancing cell growth and cell death as the levels of heat
shock proteins vary in response to environmental stress or
Key words: Signal transduction, Hsp70, Hsp90, Bag1, Nuclear
hormone receptors, Kinases, Molecular chaperones, Protein folding,
Chaperoning signaling pathways: molecular
chaperones as stress-sensing ‘heat shock’ proteins
Ellen A. A. Nollen and Richard I. Morimoto*
Department of Biochemistry, Molecular Biology and Cell Biology, Rice Institute for Biomedical Research, Northwestern University, Evanston,
IL 60208, USA
*Author for correspondence (e-mail: email@example.com)
Journal of Cell Science 115, 2809-2816 (2002) © The Company of Biologists Ltd
and transformed cell lines and range from 10 µM-100 µM for
Hsp70 and 10 µM to 150 µM for Hsp90 (C. Schmidt and
R.I.M., unpublished). Hsp70 and Hsp90 represent two different
classes of molecular chaperone. Whereas Hsp70 holds
unfolded substrates in an intermediately folded state to prevent
irreversible aggregation and catalyzes the refolding of unfolded
substrates in an energy- and co-chaperone-dependent reaction,
Hsp90 appears to interact with intermediately folded proteins
and to prevent their aggregation but lacks the ability of Hsp70
to refold denatured proteins. In vitro chaperone assays using
chemically denatured proteins have shown that Hsp90 is highly
efficient in preventing protein misfolding and that the Hsp70
chaperone machinery is required to fold the Hsp90-released
intermediates to the native state (Freeman and Morimoto,
1996; Jakob et al., 1995; Wiech et al., 1992).
Biochemical activities of Hsp70
Hsp70 interacts with co-chaperones through the N-terminal
ATPase domain and with substrates at the C-terminal substrate-
binding domain (Fig. 1a). Binding of exposed stretches of
hydrophobic residues in unfolded or partially unfolded proteins
is regulated by ATP-hydrolysis-induced conformational
changes in the ATPase domain of Hsp70, which is stimulated
by the co-chaperone Hsp40 (Hdj-1). Release of substrates
requires the binding of ATP to Hsp70, after which the
substrates either enter a new cycle of binding and release or
fold into their native conformation (Freeman et al., 1995;
Minami et al., 1996; Szabo et al., 1994).
Co-chaperone interactions can influence the Hsp70-substrate
binding-and-release cycle by stimulating, inhibiting or altering
the trafficking of Hsp70-interacting substrates. Hip binds to the
ATPase domain and increases the chaperone activity of Hsp70
by stabilizing the ADP state, which is the substrate-bound state
of Hsp70 (Fig. 1a) (Hohfeld et al., 1995). Bag1, by contrast,
inhibits the chaperone activity of Hsp70 in part by accelerating
nucleotide exchange, which affects the premature release of the
unfolded substrate (Bimston et al., 1998; Hohfeld and Jentsch,
1997; Takayama et al., 1997; Zeiner et al., 1997). Hip and Bag1
bind to Hsp70 at the same site on the ATPase domain and
directly compete to influence Hsp70 chaperone activity (Fig.
1a) (Hohfeld and Jentsch, 1997; Nollen et al., 2001). The
biological role of these co-chaperones in the regulation of
Hsp70, however, is not well understood. In part, this is because
the levels of Hip and Bag1 are approximately 1% of Hsp70, and
biochemical studies have shown that Bag1, for example, affects
the Hsp70 chaperone activity in a 1:1 molar ratio (Nollen et al.,
2000; Takayama et al., 1997). This would indicate that Bag1
influences only a fraction of the Hsp70 molecules in the cell
and therefore is unlikely to be an essential co-chaperone of
Hsp70. Bag-1 and Hip also interact with other proteins in the
cell. Bag1, for example, interacts with and influences the
function of many key components of cell death and signal
transduction pathways, including the anti-apoptotic protein Bcl-
2 and the growth regulator Raf-1 (Takayama and Reed, 2001;
Wang et al., 1996). We propose that the regulation of Bag1
function by Hsp70 serves as a checkpoint to regulate growth
and death when the levels of Hsp70 in the cell rise in response
to stress (Song et al., 2001).
Biochemical activities of Hsp90
Hsp90 substrates are less well understood; however, genetic
data support a role for Hsp90 interaction with signaling
molecules and components of cell death pathways (Pratt and
Toft, 1997). A consensus substrate recognition sequence for
Hsp90 has yet to be identified, and Hsp90 is not required for
de novo protein synthesis (Nathan et al., 1997). Common to
many of the Hsp90 partner proteins is that they are regulated,
either negatively or positively, by chaperone interaction. The
substrates also have in common the fact that they are short lived
and prone to alternative conformations. Binding of Hsp90
to these substrates prevents degradation and changes their
conformations, as suggested by their altered proteolytic
sensitivity, enzymatic activity, and, in the case of hormone
receptors, their ability to bind ligands (Pratt and Toft, 1997).
The biochemical mechanism of Hsp90-substrate interactions
is currently a topic of active investigation, and, to date, has only
partially been characterized. Although Hsp90-substrate
binding is ATP independent, substrate release requires ATP
hydrolysis (Panaretou et al., 1998; Prodromou et al., 1997).
The Hsp90-substrate binding-and-release cycle is regulated by
sequential interactions of Hsp90 with the co-chaperones Hop
and p23. Hop is a tetratricopeptide repeat (TPR)-domain-
Journal of Cell Science 115 (14)
Fig. 1. Schematic and linear
representation of the domain
structure and co-chaperone-
binding sites of Hsp70 and
Hsp90. The amino-acid residue
numbers, domains and co-
chaperone-binding sites are
indicated. (A) Domain structure
of human Hsp70. The C-
terminal EEVD motif is
characteristic for cytosolic
Hsp70s and is involved in
binding of TPR-domain-
(B) Domain structure of human
Hsp90α. The C-terminal
MEEVD motif is involved in binding of TPR-domain-containing proteins. Binding sites of co-chaperones on Hsp90 are a composite of sites of
interaction for mammalian and yeast Hsp90 [adapted from previous reports (Chen and Smith, 1998; Grammatikakis et al., 1999; Young et al.,
2811 Chaperoning signaling pathways
containing protein that binds to the C-terminal domain of
Hsp90. Binding of Hop induces a conformational change in the
ATPase domain of Hsp90 that inhibits the ATPase activity
(Prodromou et al., 1999). Dissociation of the Hop and Hsp70
from Hsp90, by an as yet unknown mechanism, results in a
conformational change in the ATPase domain that enables
binding of ATP and a transient dimerization of the N-termini
of Hsp90 molecules, whereas their C-terminal domains remain
dimerized. In turn, this allows for association with p23 and
members of the immunophilin family, followed by ATP
hydrolysis and opening of the molecular clamp formed by the
N-terminal ends of the Hsp90 dimer to release the substrate.
The Hsp90-specific inhibitor geldanamycin competes with
ATP at the ATP-binding site and thus prevents completion of
the interaction cycle by interfering with the association of p23
and ATP hydrolysis (Prodromou et al., 2000; Prodromou et al.,
1999; Young and Hartl, 2000). Hsp90 also associates with
other co-chaperones besides Hop and p23, including other
TPR-domain-containing proteins and Cdc37 (Table 1). Many
of these co-chaperones compete for binding to the C-terminal
domain of Hsp90 (Fig. 1b), which suggests that co-chaperones
might play a role in targeting Hsp90 to specific substrates.
Interaction between chaperones and signaling
The ability of a cell to know whether to grow, divide,
differentiate or die depends upon extracellular signals and the
ability to properly recognize and respond to these signals. The
cell may receive these extracellular signals in different forms
such as soluble hormones, small peptides, or proteins attached
to neighboring cells. Cellular receptors receiving these signals
transmit the extracellular information to the nucleus through
cascades of protein-protein interactions and biochemical
reactions. Chaperones of the Hsp90 and Hsp70 family and their
co-chaperones interact with a growing number of signaling
molecules, including nuclear hormone receptors, tyrosine- and
serine/threonine kinases, cell cycle regulators and cell death
regulators (Table 1). Although the functions of many signaling
molecules have been studied in detail, the significance of their
interaction with chaperones and co-chaperones remains
Below we describe two examples of signaling routes in
which chaperones of the Hsp90 and Hsp70 families play a role:
the Ras/Raf-1 signal transduction pathway and the nuclear
hormone aporeceptor complex assembly. Along with these
examples we describe what is known about the role of
chaperones and co-chaperones in these processes.
Chaperones and the Ras/Raf-1 signal transduction
The Ras/Raf-1 signaling pathway has an important role in
proliferation, cell differentiation, growth arrest and death
(Campbell et al., 1998; Kolch, 2000). In response to
extracellular signals, Ras is activated via the activities of
membrane-bound receptors and a set of modulators. Activated
Ras targets a Raf-1 heteromeric complex to the membrane,
where Raf-1 is activated by a myriad of activating proteins in
a process that is poorly understood. Upon activation, Raf-1
kinase initiates a cascade of downstream reactions ultimately
leading to phosphorylation of the ‘extracellular signal
regulated kinases’ ERK1 and ERK2 that regulate transcription
factors in the nucleus. The response depends on the cell type
and the duration and strength of the external stimulus
(Campbell et al., 1998; Kolch, 2000; Sternberg and Alberola-
Hsp90 has been found in association with many components
and regulators of the Ras/Raf-1 pathway, including Raf-1, Ksr-
1, Akt and Src (Kolch, 2000; Sato et al., 2000; Stewart et al.,
1999; Whitesell et al., 1994). Genetic studies in Drosophila
and biochemical studies in mammalian cells have
demonstrated that interactions between Hsp90 and Raf-1 are
essential for Raf-1 activation. Mutations in Drosophila Hsp90
that decrease the interaction between Raf-1 and Hsp90
suppress Raf-1 gain-of-function mutants, which affect eye
Table 1. Examples of Hsp90 and Hsp70 co-chaperones, interacting molecules and cellular function
Preferred interaction partner
Cutforth and Rubin, 1994; Gerber et al., 1995;
Grammatikakis et al., 1999; Kimura et al., 1997;
Perdew et al., 1997; Schutz et al., 1997
Silverstein et al., 1997 Hormone responsePP5 (TPR)
Progesteron/ glucocorticoid receptor
Silverstein et al., 1997
Davies et al., 2002; Silverstein et al., 1997
Davies et al., 2002; Silverstein et al., 1997
Pandey et al., 2000
Zou et al., 1998
Young et al., 1998
Prapapanich et al., 1996
Hsp70 Signal transductionBag1
Growth factor receptors
Wang et al., 1996
Bardelli et al., 1996
Zeiner and Gehring, 1995
Prapapanich et al., 1996
Takayama et al., 1995
Beere et al., 2000
Abravaya et al., 1992
Chen and Smith, 1998
Caplan et al., 1995; Dittmar et al., 1998; Kosano et al., 1998
development. The same mutations in Hsp90 that suppress
activated Raf-1 also reduce the biochemical activity of
activated Raf-1 (Cutforth and Rubin, 1994; van der Straten et
Similarly, prolonged exposure of mammalian cells to the
Hsp90 inhibitor geldanamycin dissociates Raf-1-Hsp90
complexes, resulting in decreased Raf-1 activity owing to
enhanced degradation of the Raf-1 protein (Schneider et al.,
1996; Stancato et al., 1997). By contrast, short exposures to
geldanamycin leads to Raf-1 activation, suggesting that the
transient release of Hsp90 is essential for activation. One way
to interpret the duration-dependent effects of exposure to
geldanamycin is that Hsp90 is required for maturation and
maintenance of the stability of Raf-1 but is released for activation
of Raf-1 by other regulators. Hsp90 release may even be
essential for kinase activation; this has been suggested by
analogous studies on PKR, an interferon-induced serine/
threonine kinase. Hsp90 binding, although required for adoption
of a conformation primed for activation, inhibits PKR activity
by binding to the kinase domain and regulatory domain
simultaneously (Donze et al., 2001). Deletion of the N-terminal
domain of Raf-1 results in constitutive activation, suggesting that
a conformational change of Raf-1 is required. Altogether, these
studies suggest that binding of Hsp90 is essential for maturation
of Raf-1 but that it needs to be released for activation (Fig. 2).
Hsp90 binding to Raf-1 also requires the interaction with the
Hsp90 co-chaperone Cdc37. Genetic studies in Drosophila and
gene-transfer-mediated overexpression studies in mammalian
cells have shown that Cdc37 is essential for Raf-1 activation.
Mutations in Cdc37 have effects on Drosophila eye
development owing to impaired signaling through the MAPK
signaling pathway, whereas overexpression of Cdc37 leads to
dose-dependent activation of the wild-type but not a
constitutively active form (Y340D) of Raf-1 (Cutforth and
Rubin, 1994; Grammatikakis et al., 1999). Expression of a C-
terminal deletion mutant of Cdc37 that can no longer bind to
Hsp90 inhibits activation of Raf-1 by Src and Ras, indicating
that an interaction between Cdc37 and Hsp90 is essential
(Grammatikakis et al., 1999). On the basis of these studies,
Cdc37-Hsp90 complexes have been suggested to potentiate
Raf-1 activation by making Raf-1 accessible to activation by
tyrosine kinases and other regulatory molecules that promote
Additionally, the Hsp70 co-chaperone Bag1 has been shown
to activate Raf-1 through interaction with its kinase domain
(Wang et al., 1996). Binding and activation by Bag1 requires
the C-terminal domain of Bag1, which is also necessary for
binding to Hsp70, but does not require the N-terminal
ubiquitin-like domain (Wang et al., 1996). Activation by Bag1
can bypass the effects of overexpression of a dominant-
negative form of Ras1, indicating that Bag1 acts independently
of Ras (Song et al., 2001). In contrast to Cdc37 and Hsp90, it
is not known whether Bag1 is essential for Raf-1 activation and
through what mechanism Bag1 activates Raf-1. On the basis
of mutational studies and the proposed mechanisms of
activation by other interaction partners, Bag1 could activate
Raf-1 in various ways: (1) recruitment of Raf-1 to the
membrane, similar to Ras activation of Raf-1; (2) changing the
conformation of the regulatory loop, as proposed for
phosphorylation mutants of Raf-1 that are independent of Ras
for their activation; or (3) stabilization of the active
conformation (Chong et al., 2001).
Alternatively, activation of Raf-1 may require release of
Hsp90. Therefore, it is possible that Bag1 binding could either
displace or replace Hsp90 to stabilize the active conformation
of Raf-1 (Fig. 3). Consistent with this suggestion, naturally
occurring variations in the expression levels of Hsp70 may
serve to inactivate Raf-1 when Hsp70 forms complexes with
Bag1, thus preventing its interaction with Raf-1 (Song et al.,
2001). Altogether, chaperones and co-chaperones are essential
for the maturation, stabilization and activation of the Raf-1
kinase. Although their biochemical roles in these processes
remain to be further characterized, these may be similar to their
well established roles in steroid aporeceptor complex
formation as described below.
Chaperones and nuclear hormone aporeceptor
Nuclear hormone receptors function as transcriptional
regulators that are activated in response to binding of their
specific hormone ligand. Multiple studies have shown that
interactions between Hsp90 and Hsp70 and their respective co-
chaperones is essential for activation of the nuclear hormone
receptors (Arbeitman and Hogness, 2000; Picard et al., 1990;
Pratt and Toft, 1997). The requirement for Hsp70 in receptor
activation has recently been shown genetically for the ecdysone
receptor in Drosophila (Arbeitman and Hogness, 2000).
Most of our knowledge of interactions between chaperones
and nuclear receptors comes from biochemical studies on the
Journal of Cell Science 115 (14)
Fig. 2. A model for chaperone
and co-chaperone interactions
with Raf-1. Chaperones and co-
chaperones play a role in the
maturation, activation and
inactivation of Raf-1. During
maturation Cdc37 (37) and
Hsp90 (90) bind to the regulatory
and kinase domains of Raf-1 (in
white) and keep it in an inactive conformation. Dissociation of
Cdc37 and Hsp90 followed by association of Ras and/or Bag1 leads
to a conformational change of Raf-1 that enables its activation.
Sequestration of Bag1 by Hsp70 (70) and association of 14-3-3 leads
to the inactivation of Raf-1. Raf-1 co-associating and regulatory proteins other than Ras, chaperones or co-chaperones are omitted from the
model for the sake of simplicity (see text for details and References).
2813Chaperoning signaling pathways
glucocorticoid and progesterone receptors. In an inactive
conformation, these receptors are in a heteromeric complex
with Hsp90 and Hsp90-associating proteins. Before receptors
are bound to Hsp90, they go through several steps of chaperone
and co-chaperone interactions, which have also been shown to
be essential by genetic studies in yeast (Chang and Lindquist,
1994; Chang et al., 1997; Dittmar et al., 1998; Duina et al.,
1996). Initially the steroid aporeceptor interacts with Hsp70
and Hsp40. From this Hsp70-bound state, the aporeceptor is
transferred to Hsp90 by the Hsp70- and Hsp90-binding co-
chaperone Hop (Chen and Smith, 1998). Release of Hop, ATP
binding and binding of the Hsp90 co-chaperone p23 then leads
to the formation of the final aporeceptor complex that contains
an Hsp90 dimer, p23 and immunophilins (Fig. 3) (Pratt and
Interaction with chaperones has been suggested to be
required for stabilization of the aporeceptor conformation.
Inactivation of Hsp90 by geldanamycin, for example, leads
to an increase in protease sensitivity and degradation of
glucocorticoid and progesterone receptors. Upon association
of hormone with the aporeceptor complex, the chaperone
complex is dissociated, and the receptor conformation
transitions from a labile, open structure to the stable compact
DNA-binding state that can exist independently of chaperone
complexes (Pratt and Toft, 1997).
In addition to Hsp70 and Hsp90 complexes, other proteins,
including the Hsp70-co-chaperones Hip and Bag1, have been
found associated with hormone receptors (Prapapanich et al.,
1996; Zeiner and Gehring, 1995). The role of Hip in receptor
function, apart from its interaction with the aporeceptor
complex, has not been well characterized. More is known about
the role of Bag1 with respect to its interaction with hormone
receptors. Bag1, also identified as RAP46 (for receptor
associated protein 46), interacts with many nuclear hormone
receptors, including the glucocorticoid receptor, the estrogen
receptor and retinoic acid receptor (Liu et al., 1998; Zeiner and
Gehring, 1995). Unlike Hsp70 and Hsp90 complexes, which
bind to the inactive receptor and are released after activation,
Bag1 appears to bind only to the activated receptors and after
release of HSP complexes (Zeiner and Gehring, 1995).
Overexpression studies in mammalian cells indicate that Bag1
co-migrates with the bound receptor to the nucleus and
negatively regulates DNA binding and transactivation by the
glucocorticoid receptor and prevents glucocorticoid-induced
apoptosis (Kullmann et al., 1998; Schneikert et al., 1999). This
activity is specific for two of the four isoforms of Bag1, the 46
kDa and the 50 kDa isoforms, which contain an N-terminal
domain with a repeated [EEX4] motif and are expressed in the
nucleus of mammalian cells. In addition, it requires the C-
terminal Bag1 domain that is involved in binding to Hsp70,
which suggests a role for Hsp70 in at least two steps of
hormone receptor function (Schneikert et al., 2000). In
complex with Hsp40 and Hop, Hsp70 functions in the cytosol
to regulate aporeceptor complex formation, and together with
Bag1 in the nucleus, it has a role in downregulation of the
activated receptor (Schneikert et al., 2000). Taken together,
these observations indicate that chaperone and co-chaperone
interactions with hormone receptor are involved in both
positive and negative regulation of hormone receptor activities
in a temporally and spatially restricted manner.
In addition to components of the Ras/Raf-1 signaling
pathway and hormone receptors, a wide variety of other
molecules interact with Hsp70 and Hsp90 chaperone
complexes, including the heat shock transcription factor, Apaf-
1, CFTR and Hepatitis B viral reverse transcriptase (Abravaya
et al., 1992; Beere et al., 2000; Hu and Seeger, 1996; Loo et
al., 1998; Pandey et al., 2000; Zou et al., 1998). Although little
is known about the molecular regulation and role of chaperones
in these processes, mechanisms similar to the ones used for
Raf-1 and hormone receptor regulation may be used, perhaps
in collaboration with additional, yet to be identified co-
Perspectives: consequences of variations in levels
of chaperones and co-chaperones
As exemplified above for Raf-1 kinase and nuclear hormone
receptors, the specificity of interactions between HSPs and
substrates can be determined by co-chaperones. The levels and
relative abundances of HSPs and the various co-chaperones
vary widely among cells and tissues. This variation may
provide each cell with a tailored chaperone network to support
a cell-specific response to combinations of intra- and
Fig. 3. A model for chaperone and co-chaperone interactions with nuclear hormone receptors. Chaperones and co-chaperones play a role in the
maturation and inactivation of nuclear hormone receptors. During maturation, Hsp70 (70) and Hsp(40) bind to the hormone-binding domain
(HBD) of the receptor (in white), which is followed by the association of Hop and Hsp90 (90). Maturation of the aporeceptor complex is
completed by dissociation of Hsp40, Hsp70 and Hop followed by association of p23 (23) and one of the immunophilins (I). Hormone binding
and dissociation of Hsp90, p23 and the immunophilin changes the conformation of the receptor, which then translocates to the nucleus and
activates transcription. Bag1 can associate with the activated receptor and inhibit the activities of the receptor for which it requires its Hsp70-
binding domain. AD, activation domain; DB, DNA-binding domain (see text for References).
Several physiological, pathophysiological and environmental
conditions, including development, aging, fever and several
neurodegenerative diseases often associated with the
accumulation of unfolded or misfolded proteins result in
elevated expression of all HSPs and many but not all co-
chaperones (Morimoto, 1998). Under such conditions, it may be
less important that a particular unfolded polypeptide is
associated with a specific chaperone; rather it is the conserved
‘holding’ function of chaperones that is essential, with triage
decisions on the fate of these substrates being determined later
As a consequence of these changes, however, the
equilibrium between substrates, HSPs and co-chaperones is
likely to be disturbed, which has potentially profound
consequences for the phenotype of the cell. Changes in
the abundance and relative levels of chaperones and co-
chaperones could result in novel combinations of HSPs,
which, in turn, could redirect information flow through the
intracellular pathways and change the overall response to
signals. Whereas some pathways may become favored
because of an increase in the level of a particular co-chaperone
that is specifically required for its regulation, other pathways
might be suppressed or acquire constitutive activation by the
lack of critical chaperone components. The overall effect of
changes in chaperone or co-chaperone levels on cellular and
organismal phenotypes probably depends on which chaperone
or co-chaperone is affected. For example, changes in the levels
of Hsp90 by exposure of cells or organisms to geldanamycin
on in Drosophila by altering gene dosage, and Hsp70 by
mutation or overexpression, have pleiotropic and often more
severe consequences than do changes in the leels of a co-
chaperone that is specific for a subset of substrates, such as
Cdc37, which preferentially interacts with kinases. One
example of a change in signaling as a consquence of altered
levels of HSPs is the inhibition of the Ras/Raf-1 signaling
pathway in tissue culture cells when the levels of Hsp70
increase in response to stress. The increased levels of Hsp70
sequester Bag1, which disrupts the stimulatory properties of
Bag1 on Raf-1, which then results in cell growth arrest (Song
et al., 2001).
HSPs have co-evolved as integral components of signal
transduction networks, in which they can function in the
maturation, activation and inactivation of signaling molecules.
Their involvement in a particular pathway within the network
is determined by the availability and relative abundance of
partner-specific co-chaperones, which will influence, in a cell-
type-specific manner the natural response to physiological
intracellular and extracellular signals. Consequently, we
suggest that altered levels of HSPs and co-chaperones in
response to stress or disease states alters how organisms
integrate and respond to the flow of their normal physiological
signals. Future studies in multicellular model systems will help
to elucidate with greater detail the molecular basis for the
pervasive role of molecular chaperones in organismal
development and disease and how they respond to altered
chaperone and co-chaperone levels associated with fluctuating
environmental conditions and disease.
We thank Sri Bandhakavi and Marija Tesic for critical reading of
the manuscript. E.N. was funded by a postdoctoral fellowship from
the Netherlands Organization for Scientific Research and an EMBO
long-term fellowship. R.M. is supported by NIHGM38109 and the
Huntington Disease Society of America Coalition for the Cure.
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