(Received for publication, February 19, 1997)
From the McArdle Laboratory for Cancer Research, University of Wisconsin Medical School, Madison, Wisconsin 53706-1599
In an effort to identify regulators of aryl
hydrocarbon receptor (AHR) signaling, we have employed the yeast
two-hybrid system to screen for human proteins that interact in a
ligand-dependent manner with the AHR. After screening
1.4 × 106 clones from a human B cell library,
two distinct clones were identified that associated specifically with
the liganded receptor. No clones were identified that interacted
preferentially with the unliganded AHR. One of the
ligand-dependent clones, ARA9, encodes a novel 330-amino
acid protein with regions of amino acid sequence similarity to the
52-kDa FK506-binding protein known to be associated with the
glucocorticoid receptor. Yeast two-hybrid experiments with ARA9
demonstrated a strong interaction with the AHR that is enhanced 11-fold
in the presence of the ligand -naphthoflavone. In vitro
experiments using proteins generated in reticulocyte lysates confirmed
this interaction and indicated that ARA9 can be co-immunoprecipitated
with the AHR using antisera raised specifically for either the AHR or
the 90-kDa heat shock protein. The observation that ARA9 has a high
affinity for both the 90-kDa heat shock protein-associated and
ligand-activated forms of the AHR suggests that ARA9 is a component of
the AHR-signaling pathway in vivo.
The AHR1 is a ligand-activated
transcription factor that regulates the expression of xenobiotic
metabolizing enzymes in response to binding environmental pollutants
such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (1). The AHR is
a member of the Per-ARNT-Sim homology domain superfamily of regulatory
proteins that also includes the AHR's dimer partner, ARNT, and such
proteins as HIF1 and Sim (reviewed in Ref. 2). Members of this
family are distinguished by a region of similarity of approximately 200 amino acids termed PAS (3). In the AHR this domain is involved in
dimerization with other PAS-containing proteins, ligand binding, and
association with hsp90 (4-6). Most members of this superfamily also
contain a basic helix-loop-helix domain immediately N-terminal to the
PAS region (2). The basic domain mediates the recognition and binding of these factors to specific DNA sequences in enhancer elements that
regulate transcription of target genes (6, 7). The helix-loop-helix
domain functions as a primary dimerization surface that directs
interactions with appropriate dimeric partners (8, 9).
Although no obvious structural relationship is apparent, the AHR and certain members of the steroid receptor superfamily exhibit similarities in their signaling mechanism (10, 11). Biochemical studies have indicated that the unliganded AHR and the GR are located in the cytosol in a complex with a dimer of the molecular chaperone hsp90 and other cellular proteins (12-14). hsp90 has been shown to be an important regulator of receptor activity. Genetic studies in yeast systems deficient in hsp90 have demonstrated an absolute requirement for hsp90 in both glucocorticoid and AHR signaling (10, 11, 15). Biochemical studies have correlated the association of hsp90 with an increase in ligand binding capacity, and dissociation of hsp90 has been correlated with an increase in DNA binding capacity (16-19). Finally, current models of signaling for the GR and AHR are quite similar. In response to ligand binding in the cytosol, receptor-hsp90 complexes translocate to the nucleus where hsp90 dissociates and the activated GR homodimerizes or the AHR heterodimerizes with ARNT. The resulting complexes attain binding specificity for their cognate enhancer elements to regulate transcription of specific batteries of genes.
In addition to hsp90, a number of other intracellular proteins appear to play a role in GR signaling. The GR has been shown to exist in a complex with two other heat-inducible proteins, hsp70 and FKBP52 (20). hsp70 binds to the hormone binding domain of the GR and may be required for hsp90 to bind to the receptor (21). FKBP52 is an immunophilin of the FK506 binding class that binds directly to hsp90 in the non-liganded receptor heat shock protein complex and may play a role in targeted protein movement (20-24). Other proteins that have been recovered in native steroid receptor complexes bound to hsp90 include a highly acidic 23-kDa protein and an immunophilin of the cyclosporin A binding class known as Cyp-40 of unknown function (20).
In contrast to GR, less is known about the composition of the cytosolic and ligand-activated forms of the AHR. Chemical cross-linking experiments have revealed the presence of a 46-kDa protein of unknown function in the AHR cytosolic complex (25-27). Purification of ligand-activated AHR complexes using DNA affinity chromatography has indicated that the DNA-bound form of the AHR is found associated with ARNT and a number of unknown proteins including those with apparent molecular masses of 110, 57, and 54 kDa (28, 29). In an effort to identify additional proteins that are part of the AHR complex or that may regulate its activity, we used the yeast two-hybrid system to screen for proteins that associate with the AHR in a ligand-dependent manner in vivo and then confirmed these interactions by co-immunoprecipitation assays in vitro. We report here the isolation of a cDNA clone encoding a protein whose interaction with the AHR in vivo is detected only in the presence of ligand and with sequence similarity to the FKBP52 protein found associated with the GR.
The oligonucleotide sequences are: OL176, CGGGATCCTTACACATTGGTGTTGGTACAGATGATGTAGTC; and OL226, CGGGATCCTCATGGCGGCGACTACTGCCAACC.
Strains and PlasmidsSaccharomyces cerevisiae
strain L40 (MAT a, his3200, trp1-901, leu2-3,
112, ade2,
LYS2::(lexAop)4-HIS3,
URA3::(lexAop)8-lacZ, gal80) was used for two-hybrid screens and interaction assays (30). Plasmid pBTM116 is a 2-µm TRP-marked plasmid
containing the full-length Escherichia coli LexA protein
under the control of the ADH1 promoter for expressing LexA
fusion proteins (31). pGAD424 is a 2-µm LEU-marked plasmid
for making fusions with the GAL4 transcriptional activation
domain (32). Plasmid pBTMAHR
TAD contains the murine AHR lacking
amino acids 494-640 fused in frame to the full-length E. coli LexA protein (33). pBTMARNTC
325 was constructed by
amplifying amino acids 1-464 from pSPORTARNT (4) with the primers
OL176 and OL226 using standard polymerase chain reaction protocols as
described (10). The amplified fragment was ligated into the vector
pGEM-T (Promega, Madison, WI). The resulting plasmid pGEMARNTC
325
was digested with BamHI, and the insert was subcloned into
the BamHI site of pBTM116.
Transformation of
plasmids into the strain L40 was performed by a modified lithium
acetate protocol as described (10, 34). For library screening, the
transformants were replica-plated after 4 days to fresh plates of the
same media and incubated for 2 days. The colonies were again
replica-plated to fresh plates that contained either no agonist or 1 µM NF, then incubated at 30 °C for 2 days, and
assayed for lacZ expression as described (35).
Fifty nanograms of ARA9 and 25 ng of actin cDNA were random-primed and used as a probe to hybridize human multiple tissue and human immune system Northern blots (CLONTECH, Palo Alto, CA) as described (33).
QuantitativeS.
cerevisiae strain L40 was transformed with the appropriate
plasmids and plated on selective media either treated by spreading 100 µl of 1 mM NF over the surface (78.5 cm2)
of the medium or left untreated. The plates were incubated for 4 days
at 30 °C. For each assay, three colonies were suspended in 100 µl
of buffer Z (60 mM Na2HPO4, 40 mM NaH2PO4, 10 mM KCl, 1 mM MgSO4, 35 mM
2-mercaptoethanol). Five microliters of the cell suspension were
diluted in 170 µl of buffer Z, and the A600 was measured to estimate cell density. The remaining cell suspension was pelleted by centrifugation, and
-galactosidase activity was determined by a chemiluminescent reporter assay protocol
(Galacto-light, Tropix, Bedford, MA). Light units were divided by the
A600 to normalize values to cell density.
AHR, ARNT, and ARA9 were
expressed in vitro using the TNT-coupled rabbit reticulocyte
lysate (Promega) using the plasmids pSportAHR, pSportARNT, and PL580 as
templates, respectively (4). In our hands, this expression system
typically produces about 10 fmol of 35S-labeled translated
protein per 50 µl of reaction mixture (4). Associations were formed
by mixing 5 µl of the appropriate in vitro translated
proteins in a 1.5-ml microcentrifuge tube, and co-immunoprecipitations
were performed as described previously (33). For
co-immunoprecipitations using the AHR, 0.1 µl of 1 mM
NF or Me2SO was first added to the reaction and
incubated for 2 h at 30 °C.
To identify novel factors
involved in AHR signaling, we designed a two-hybrid assay that would
detect proteins that interact with the AHR in a
ligand-dependent manner. A plasmid containing a LexA-AHR
fusion that had a deletion in the transcriptional activation domain
(LexA-AHRTAD (36)) was used to screen a human B lymphocyte cDNA
library that was fused to the GAL4 transcriptional activation domain
(6, 37). The transformants were plated on selective media containing 1 µM AHR agonist
NF or onto untreated plates. The
surviving colonies from each group were twice replica-plated onto
agonist and nonagonist plates and assayed for both histidine prototrophy and lacZ expression exclusively under either
agonist or nonagonist conditions. 700,000 clones were screened under
each condition. No clones were identified that interacted
preferentially with the unliganded AHR. However, we isolated 13 positive colonies that interacted with AHR in a
ligand-dependent manner. Ten of these clones retained
ligand-dependent interactions with the LexA-AHR
TAD construct upon secondary screening against the unrelated protein, lamin
(38). Sequence analysis revealed that five clones represented a
distinct cDNA of 2.2 kilobase pairs, designated ARA3 (for
eceptor-ctivated), and the
remaining five clones represented a second cDNA of 1.03 kilobase
pairs that was designated ARA9. In this paper, we describe the
characterization of the ARA9 cDNA.
The original ARA9 clone,
pbARA9, contained an open reading frame of 325 amino acids and lacked a
Kozak consensus methionine (39). To obtain additional upstream sequence
information, expressed sequence tags were identified by a BLAST search
of the GenBankTM data base (33). Three of the corresponding cDNAs
were obtained from the IMAGE consortium at Washington University and
subjected to sequence analysis. One of the expressed sequence tag
clones, PL580 (GenBank accession number R50134[GenBank]), contained the entire
ARA9 sequence as well as additional 3-untranslated sequences, a
poly(A) tail, and an additional 131 nucleotides at the 5
end for a
total of 1244 nucleotides (Fig. 1A). This
clone also contained a Kozak methionine with an upstream in-frame stop
codon at position
99 (39). The ARA9 mRNA is approximately 1.35 kilobases in length and is expressed in all tissues examined with
higher levels of expression in heart, placenta, and skeletal muscle
(Fig. 2).
The DNA and deduced amino acid sequence of ARA9 is shown in Fig.
1A. It contains an open reading frame encoding a novel
protein of 330 amino acids followed by a short 3-untranslated region ending in a polyadenylation sequence. The deduced amino acid sequence has a predicted molecular weight of 38, which is consistent with the
estimated molecular mass of 37 kDa as determined from the electrophoretic mobility of in vitro translated ARA9 on
SDS-polyacrylamide gel electrophoresis (Fig. 4). The N-terminal portion
of ARA9 contains a region of 79 amino acids, which is 30% identical to
a region in FKBP52 and 28% identical to FKBP12. A second region of 11 amino acids is 63% identical to a region in FKBP12 (Fig.
1B). The C terminus harbors three regions among residues
182-215, 234-267, and 268-301 that conform to the consensus TPR
domain and are 21, 24, and 47%, respectively, identical to TPR domains
found within FKBP52. TPR domains are highly degenerate 34-amino acid
sequences containing two regions able to form short amphipathic
-helices that are thought to mediate protein-protein interactions in
a number of proteins (40, 41). These domains have been identified in
many proteins involved in such diverse functions as cell division in
yeast, protein import, and Drosophila development (42).
Interaction of the AHR and ARNT with ARA9
To obtain
quantitative data describing the ligand-dependent
interaction of the AHR with ARA9 and to determine whether ARA9 interacts with the structurally related AHR dimeric partner, ARNT, LexA
fusions of AHRTAD or ARNTC
325 were co-transformed with pbARA9
into the strain L40, plated on selective media, and
-galactosidase activity measured as an indication of the relative strength of the
interactions in the presence or absence of
NF (for AHR
TAD). In
the presence of ARA9, induction of
-galactosidase expression was
increased 25-fold over control in the absence of agonist, and this
interaction is enhanced approximately 11-fold in the presence of
NF.
These experiments demonstrate that ARA9 interacts with the AHR in a
conditional manner (Fig. 3). Co-transformation of
LexAARNTC
325 with ARA9 only weakly induced
-galactosidase activity (2.7-fold over control) suggesting that ARNT may not associate
with ARA9 in vivo or that this interaction is much weaker than that with AHR (Fig. 3).
To further characterize the interactions of ARA9 with the AHR and ARNT, co-immunoprecipitation assays were performed using full-length in vitro translated AHR, ARNT, and 35S-labeled full-length ARA9. Using AHR antisera, 35S-labeled ARA9 could be co-precipitated with the AHR both in the presence and absence of ligand (Fig. 4, lanes 2-5). We also asked whether ARA9 was a component of the AHR·hsp90 complex. The AHR·ARA9 complex was detected using anti-hsp90 antisera, indicating that ARA9 is present in AHR·hsp90 complexes (Fig. 4, lanes 6-9). However, ARA9 could not be co-precipitated with hsp90 in the absence of AHR (Fig. 4, lanes 10 and 11). These results suggest that either ARA9 binds to the AHR directly or that stable or high affinity association with hsp90 is AHR-dependent.
The observation that ARA9-AHR interactions are
ligand-dependent in vivo and ligand-independent
in vitro suggests that ARA9 constitutively associates with
the AHR regardless of the ligand binding status of the receptor, and
that the apparent ligand dependence of the complex in vivo
reflects the change in subcellular localization from the cytosol to the
nucleus where it is able to activate transcription. This observation
also suggests that the ARA9-AHR interaction occurs at multiple stages
of the signaling pathway and that the ligand dependence of the
two-hybrid assay is a reflection of the fact that the interaction can
be maintained during nuclear translocation, hsp90
association/dissociation, dimerization, and DNA binding of the
LexA-AHRTAD chimera. The negative co-immunoprecipitation data with
ARA9 and ARNT (Fig. 4, lanes 12 and 13) supports
the in vivo experiments which demonstrated a weak
interaction between ARA9 and ARNT and indicate that ARA9 may have
marked restrictions in the number of proteins with which it can
interact. With this in mind, it is interesting to point out that our
preliminary two-hybrid data suggest that ARA9 does not interact with GR
(data not shown).
ARA9 is a candidate for the previously described 43-kDa subunit of the AHR-cytosolic complex (26). Chemical cross-linking experiments using murine Hepa1c1c7 cytosol have shown that the AHR-cytosolic complex exists as a heterotetramer (25, 26). One of these subunits is the AHR, two others have been identified as a hsp90 dimer, and the fourth is an unidentified 43-kDa subunit (26). In a previously reported study, a heteromeric intermediate of 146 kDa was isolated using immunoabsorbed anti-AHR antibody and was postulated to contain the 97-kDa AHR plus the 43-kDa-associated protein suggesting that this factor may be directly associated with the AHR (25, 26). Additionally, the 43-kDa subunit was shown not to be stably associated with cytosolic-hsp90 complexes in Hepa1c1c7 extracts (43). These findings are in agreement with the data presented here in which ARA9, a human protein of similar size to p43, appears to directly interact with the AHR, but not with cytosolic hsp90 in the absence of receptor.
In overall structure, ARA9 is related to FKBP52, also known in the literature as p59 and hsp56 (reviewed in Ref. 20) (Fig. 1B). FKBP52 is one of several chaperone proteins found in the untransformed GR heterocomplex and has an overall length of 459 amino acids (20). It binds directly to hsp90 in the heat shock complex through three TPR units located within the C-terminal half of the protein (44, 45). FKBP52 also contains two FKBP12-like domains termed I and II. Although the function of domain II is unknown, domain I has high identity (49%) with FKBP12 and has been shown to exhibit peptidylprolyl isomerase activity and bind to the immunosuppressants FK506 and rapamycin (44, 46). The biological significance of this activity is unclear since blocking the peptidylprolyl isomerase activity with FK506 does not affect the cytosolic heterocomplex assembly or proper folding of the GR into a high affinity ligand binding conformation (47). Furthermore, FKBP52 does not appear to be necessary for GR·hsp90 heterocomplex assembly (48). Recent experiments, however, have indicated that it may play a role in receptor translocation to the nucleus upon activation by ligand (23, 24).
In contrast to FKBP52, ARA9 is a smaller protein (330 amino acids) and has only one FKBP12-like domain with weaker identity to FKBP12 (28%) and three TPR domains in the C terminus. It has yet to be established if ARA9 exhibits FK506 binding and peptidylprolyl isomerase activity and how this may affect receptor function. One important difference between ARA9 and FKBP52 is that while FKBP52 has been found to be associated with hsp90 through chemical cross-linking and co-immunoprecipitation experiments, ARA9 does not appear to stably interact with hsp90 in the absence of AHR. Possible explanations for this difference in hsp90 binding affinity in the absence of AHR may be that 1) all TPR domains do not form equally tight interactions with hsp90, 2) the affinity of the ARA9-hsp90 association may be below the limit of detection in our assay, and 3) the hsp90 antibodies used in our experiments may recognize hsp90 complexes different from those used to co-precipitate hsp90-FKBP52 complexes (i.e. our hsp90 antibodies may not be able to recognize hsp90·ARA9 complexes). In any case, while ARA9 appears to be in the same class structurally as FKBP52 and although both proteins associate with nuclear receptors, their roles in receptor signaling may be distinct.
Because of its stable, ligand-dependent interaction with AHR, ARA9 is likely to have an important role in receptor function. If ARA9 and p43 are the same protein and ARA9 does indeed form part of the AHR cytosolic complex it may associate with the AHR complex and function along with hsp90 in the folding and stabilization of the receptor in an inactive form. Alternatively, since ARA9 also appears to interact with the AHR as part of a DNA-binding complex, it is possible that ARA9 functions as a co-activator forming a bridge between the receptor and basal transcription factors similar to the way that steroid receptor co-activators such as SRC-1 and Trip1 are thought to function (49, 50). Given its structural similarity with FKBP52, it is possible that ARA9 may have a similar role in AHR signaling possibly acting as a targeting molecule to direct receptor trafficking to the nucleus. Finally, we cannot rule out the possibility that ARA9 has a function unrelated to AHR signaling and that this function is dependent upon association with AHR.
The nucleotide sequence(s) reported in this paper has been submitted to the GenBankTM/EMBL Data Bank with accession number(s) U78521[GenBank].
We thank Stanley Hollenberg for the S. cerevisiae strain L40 and the plasmid pBTM116, Stephen Elledge for the B cell library, and Alan Poland for the Bear1 AHR antibody and the hsp90 antibody.