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(Received for publication, October 9, 1996, and in revised form, November 20, 1996)
From the Virus Tumor Biology Section, Laboratory of Molecular
Virology, NCI, National Institutes of Health,
Bethesda, Maryland 20892-5005
ABSTRACT We have previously shown that the parathyroid
hormone-related protein (PTHrP) promoter contains binding sites for
transcription factors Ets1 and Sp1 and that human T-cell lymphotropic
virus type I (HTLV-I) Tax cooperates with Ets1 to transactivate the PTHrP P2 promoter. Using the yeast two-hybrid interaction system, we
now provide evidence that Tax interacts with Ets1. Moreover, a double
mutation (D22A,C23S) in the Tax protein that abrogated the Tax/Ets1
interaction also inhibited the Tax/Ets1 cooperative effect, suggesting
that the interaction between Tax and Ets1 is important for
transactivation of the PTHrP promoter. In coimmunoprecipitation assays,
we find that Tax facilitates the interaction between Ets1 and Sp1,
forming a ternary complex. When the Sp1 site in the PTHrP promoter was
mutated, the Tax/Ets1 cooperative effect was dramatically decreased.
This suggests that Sp1 plays an important role in the Ets1-dependent Tax transactivation of the PTHrP P2
promoter. Finally, we demonstrate that Gal4-Tax is a strong activator
of the Gal PTHrP promoter, implying that Tax contributes directly to
the transcriptional activation of the promoter. We propose a model in
which the Tax/Ets1 cooperative effect on the PTHrP P2 promoter is based
on the ability of Tax, Ets1, and Sp1 to form a ternary complex on the
template DNA. Tax facilitates the interaction of Ets1/Sp1 and
participates directly in the transcription initiation process.
INTRODUCTION Human T-cell lymphotropic (leukemia) virus type I
(HTLV-I)1 is associated with adult T-cell
leukemia and the neurological disease tropical spastic
paraparesis/HTLV-I-associated myelopathy (1-7). The HTLV-I genome
encodes a unique protein (Tax) that is required for the virus-induced
transformation of T-lymphocytes (8-10). An important function of this
protein is to deregulate certain cellular genes including
interleukin-2, interleukin-2 receptor, c-fos, major
histocompatibility class II, and parathyroid hormone-related protein
(PTHrP) (11-17, 19-21).2 Tax exerts its
transactivating effect on promoters through its ability to interact
with and activate certain transcription factors, like the
cAMP-responsive element-binding protein (CREB), the serum response
factor, NF- We have previously shown that the transcription factor Ets1 is
important for Tax transactivation of the PTHrP P2 promoter (21). Ets1,
a member of the Ets family of transcription factors (for review, see
Ref. 43), is expressed only in certain cell types, including
T-lymphocytes. It cooperates with Sp1 to transactivate certain
promoters, including the PTHrP P2 promoter (44-46). PTHrP was
initially described as the causative agent for humoral hypercalcemia of
malignancy, a disease associated with a variety of tumors (for review,
see Ref. 47). More recently, PTHrP has also been shown to play an
important role in controlling proliferation and apoptosis in normal
cells and cancer cells (48-58). PTHrP is expressed by most
HTLV-I-transformed cells (20, 59-61). This study was undertaken to
investigate the mechanism underlying the Tax/Ets1 cooperative effect on
the PTHrP P2 promoter. We present evidence that Tax interacts with Ets1
in the yeast two-hybrid system. Moreover, a Tax mutant that disrupts
the complex decreases the functional interaction between Tax and Ets1
on the PTHrP promoter. Moreover, the interaction between Tax and Ets1
is part of a multimeric complex composed of Tax, Ets1, and Sp1.
MATERIALS AND METHODS Osteosarcoma
OsA-CL cells (62) were maintained in RPMI 1640 medium with 10% fetal
calf serum. For transient transfections, cells were trypsinized,
suspended in RPMI 1640 medium with 10% serum, centrifuged, and washed
in RPMI 1640 medium. Cells were resuspended in RPMI 1640 medium (1 × 107 cells/250 µl), mixed with CsCl-purified plasmid
DNA, and electroporated using a Life Technologies Cell-Porator (250 V,
800 microfarads). After incubation on ice for 30 min, cells were plated
on tissue cultures dishes (100 × 20 mm) in 10 ml of RPMI 1640 medium with 10% serum and incubated at 37 °C either overnight
(pCTax) or for 2 days (pSG424-Tax). Chloramphenicol acetyltransferase
activity was determined using [14C]chloramphenicol as
described previously (63) or by chloramphenicol acetyltransferase
enzyme-linked immunosorbent assay (Boehringer Mannheim).
Plasmids for the expression of wild-type Ets1,
Extracts of OsA-CL cells were
separated on a 10 or 12% SDS-polyacrylamide gel in a Bio-Rad
Miniprotean II cell, and proteins were subsequently blotted onto
Immobilon-P membrane in a Bio-Rad Trans-Blot cell. After blocking in
3% nonfat milk dissolved in blocking buffer (100 mM NaCl,
2 mM EDTA, and 10 mM Tris-HCl, pH 7.5) for 30 min, the membrane was probed with the primary antibody for 20 min,
followed by three washings in blocking buffer for 5 min each. After a
20-min incubation with an anti-IgG-horseradish peroxidase conjugate
(1:10,000 dilution in blocking buffer with 0.3% milk) that recognized
either anti-rabbit or anti-mouse IgG depending on the type of primary
antibody used, the filter was extensively washed in blocking buffer
(for ~1 h) and incubated with ECL reagent (Amersham Corp.). Bands
were visualized by using Kodak X-Omat AR film. The following primary
antibodies were used: rabbit anti-Ets1 (C-20, Santa Cruz Biotechnology
Inc.; 1:2000 dilution in blocking buffer with 1% milk), rabbit
anti-Sp1 (PEP-2, Santa Cruz Biotechnology Inc.; 1:1000 dilution in
blocking buffer with 1% bovine serum albumin), mouse anti-Tax (1:500
dilution in blocking buffer with 1% bovine serum albumin), and rabbit
anti-Gal4 DNA-binding domain (Upstate Biotechnology, Inc.; 1:2000
dilution in blocking buffer with 1% bovine serum albumin).
Ets1 baculovirus extract (0.4 µl) was
incubated with Tax baculovirus extract (0.4 µl) and/or 0.2 µl of
purified Sp1 (Promega) in a total volume of 10 µl of
immunoprecipitation buffer (0.65 mM dithiothreitol, 1 mM EDTA, 10 mM Tris, pH 7.5, 12.5 mM KCl, 1.8 mM HEPES, pH 7.3, 1 mM
MgCl2, 1 µM ZnCl2, 1 mg/ml bovine
serum albumin, 12% glycerol, and 0.06% Nonidet P-40) for 10 min on
ice. After the addition of 6 µl of anti-Ets1-agarose conjugate
suspension (Santa Cruz Biotechnology Inc.; 1:5.5 dilution in
immunoprecipitation buffer) to each sample and incubation on ice for 30 min, the agarose beads were spun down and washed twice in
immunoprecipitation buffer. Ten µl of 2 × standard protein
sample buffer was added; the mixture was boiled in water for 5 min and
centrifuged; and the supernatant was analyzed by Western blotting as
described above. In control experiments, Ets1 was not added to the
reaction mixture.
An
Ets1 gene fragment encoding amino acids 2-441 and
containing a EcoRI site and an XhoI site at the
5 RESULTS To study the mechanism underlying the cooperative
effect of Tax and Ets1 on the activity of the PTHrP P2 promoter, we
utilized the human osteosarcoma cell line OsA-CL. In this cell line,
Tax is not able to transactivate the P2 promoter containing the
We also tested a To
investigate the possibility of whether Tax could directly activate the
PTHrP P2 promoter, we tested a Gal4-Tax fusion protein for its ability
to transactivate a Gal4 PTHrP promoter (Fig.
2A). As shown in Fig. 2B, Gal4-Tax
increased the Gal4 PTHrP promoter activity by 38-fold. The Gal4-Tax
fusion protein did not activate the PTHrP promoter in the absence of
the Gal4-binding site, nor did Gal4-Tax significantly increase the
activity of the wild-type promoter in the absence of Ets1. Furthermore,
native Tax failed to activate the Gal4 PTHrP promoter. This
demonstrates that the strong activation of the Gal4 PTHrP P2 promoter
by Gal4-Tax was dependent on the Gal4 domain and the Gal4 DNA-binding
site. It is noteworthy that of the Gal4 binding site-containing
promoters we have analyzed, the Gal4 PTHrP P2 promoter was one of the
strongest responders to Tax (data not shown). The results suggest that
the Tax/Ets1 synergistic effect may be partly due to the ability of Tax
to communicate with the basal transcription machinery of the PTHrP P2
promoter, increasing Ets1-dependent transcriptional
activation of the wild-type promoter.
The data in Fig. 2 also show that, similar to
native Tax, Gal4-Tax can cooperate with Ets1 to transactivate the
wild-type PTHrP promoter. The requirement of Ets1 for Gal4-Tax-mediated transactivation of the wild-type promoter, but not the Gal4 promoter, allows screening of Tax mutants that fail to interact functionally with
Ets1. Gal4-Tax mutants that fully activate the Gal4 promoter in the
absence of Ets1, but that would be unable to transactivate the
wild-type promoter in the presence of Ets1, could be considered as
being specifically impaired in their ability to cooperate with Ets1. We
tested four deletion mutants of Tax (
To
investigate the possibility that for functional cooperativity with
Ets1, Tax must associate with Ets1, we chose the yeast two-hybrid
system for the detection of protein-protein interaction in
vivo. Tax was fused to the LexA DNA-binding domain and Ets1 to an
acidic activator. After both proteins were expressed in yeast cells
carrying a LexA-responsive
Since the M5 Tax mutation impaired the ability of Tax to functionally
cooperate with Ets1, we were interested in analyzing the effect of this
mutation on Ets1 binding in the two-hybrid system. When we coexpressed
a LexA-M5 Tax fusion protein together with either wild-type or To
analyze Tax/Ets1 binding in vitro, we performed
immunoprecipitation experiments with an anti-Ets1 antibody that can
recognize native Ets1 (70). We mixed equal volumes of Ets1 and Tax
baculovirus extract with anti-Ets1-agarose conjugate and analyzed the
washed immunoprecipitate for the presence of Tax by Western blot
analysis. The amount of Tax in the immunoprecipitate was comparable to
the background signal obtained in control reactions (Fig.
5A, lanes 3 and 6).
We have previously shown that Sp1 binds adjacent to Ets1 in the PTHrP
P2 promoter and cooperates with Ets1 to activate this promoter (46). We
were interested in analyzing whether the presence of Sp1 would affect
the outcome of the immunoprecipitation experiment. When we added Sp1 to
Tax and Ets1, we found that the amount of Tax that precipitated
together with Ets1 was greatly increased (Fig. 5A,
lane 1). When we reprobed the Western blot with anti-Sp1, a
pattern was observed that was similar to that seen with anti-Tax. With
Ets1 alone or in the absence of Ets1, little Sp1 was precipitated (Fig.
5B, lanes 2 and 5). In contrast, Sp1
binding was found to be significantly increased when both Ets1 and Tax
were present (lane 1). More important, the amounts of
antibody used in these experiments were similar as equal amounts of IgG
heavy chain could be detected in all lanes (Fig. 5B). Also,
similar amounts of Ets1 were precipitated with the Ets1 antibody beads
(Fig. 5C, lanes 1-4). These results suggest that
Sp1 stabilizes the interaction between Tax and Ets1, forming a stable
ternary complex.
Since Tax was able to form a ternary complex with Ets1
and Sp1, it was of interest to determine whether a mutation in the Sp1-binding site of the PTHrP P2 promoter would affect
Ets1-dependent transactivation of the PTHrP P2 promoter by
Tax. When tested in the presence of Ets1 alone, the Sp1 and Ets1 mutant
PTHrP P2 promoters (Fig. 6A) showed a 3-fold
decreased activity compared with the wild-type promoter (Fig.
6B). This is consistent with results obtained previously
with Jurkat cells (46). When both Ets1 and Tax were expressed, these
differences were more dramatic, as indicated by a 5.5- or 11-fold lower
activity of the Sp1 or Ets1 mutant promoter, respectively,
versus the wild-type promoter. These data show that not only
the Ets1 site, but also the Sp1 site is required for the Tax/Ets1
cooperative effect.
DISCUSSION Previous studies have demonstrated that the transcription factor
Ets1 is important for Tax transactivation of the PTHrP promoter. Our
present studies, using the yeast two-hybrid system, further demonstrate
that Tax is able to form a complex with the transcription factor Ets1
in vivo. More important, mutation of Tax at amino acids 22 and 23 (Tax mutant M5) inhibited the interaction between Tax and Ets1
and the Tax/Ets1 cooperative effect. These results suggest that the
interaction between Tax and Ets1 is important for transactivation of
the PTHrP P2 promoter. It is interesting that, although the mutations
are in close proximity, Tax mutants M17/20 and M6 did not show the
differential activity pattern on the two PTHrP P2 promoters. The
importance of specific amino acids for Tax transactivation through
NF- The immunoprecipitation results suggest that Tax, Ets, and Sp1 form a
stable ternary complex on the DNA. The indirect binding of Tax to DNA
would allow Tax to interact with basal transcription factors like TFIIA
or TFIID (41, 42) and facilitate transcription initiation. Consistent
with this hypothesis, we found that Tax is a strong activator of this
promoter once it is directly tethered to the DNA through a Gal4
DNA-binding domain. In addition, Tax binding to Ets1 may superactivate
Ets1 in that it may mimic Ras-dependent phosphorylation of
Ets1 at threonine 38 (74). Such a pathway would resemble the
CREB-dependent transactivation of the HTLV-I promoter,
where the Tax-CREB complex binds the cofactor CREB-binding protein
irrespective of CREB phosphorylation (24).
Removal of the exon VII domain of Ets1, which results in an increase in
Ets1 DNA binding activity by 10- to 20-fold in vitro (75),
did not affect Ets1 activity on the PTHrP P2 promoter. This suggests
that exon VII-regulated DNA binding activity of Ets1 is not a limiting
factor for the transactivation of the PTHrP P2 promoter, at least not
under transient transfection conditions where Ets1 is
overexpressed.
There may be several advantages for formation of the Tax-Ets-Sp1
complex. First, in complex with Tax, Ets1 and Sp1 might be protected
from the action of specific inhibitor proteins. In myelomonocytic cells, Ets1 activity is controlled by an AP1-like protein (MafB) that
directly binds to Ets1 (76). Similarly, nuclear protein p74 has been
reported to bind specifically to Sp1 and to inhibit Sp1-mediated
transactivation (77). Second, Tax association with Ets and Sp1 could
reduce the activity of inhibitory kinases that phosphorylate and
inactivate Ets1 or Sp1 (78-80). Third, as a complex, Ets1 and Sp1 may
be less likely to be replaced by related proteins that recognize the
Ets1- or Sp1-binding site (18, 81-84). Thus, Tax not only contributes
directly to the activation of the PTHrP promoter, but by complexing
with Sp1 and Ets1, assures optimal regulation of transcriptional
activity.
FOOTNOTES Acknowledgment We thank J. Ghysdael for providing the
REFERENCES
Volume 272, Number 8,
Issue of February 21, 1997
pp. 4953-4958
©1997 by The American Society for Biochemistry and Molecular Biology, Inc.
,
B, and NF-Y2 (22-32). Tax activates NF-B by
mediating its release from a complex with the inhibitor IB (33-36).
It increases the activity of CREB by facilitating its dimerization
(25-28) and its binding to the cofactor CREB-binding protein (CBP)
(24). Tethering to DNA through a transcription factor would allow Tax
to communicate with the basal transcription machinery. Consistent with
this hypothesis, Tax has been shown to be capable of activating
promoters when fused to a DNA-binding domain (37-40). In addition, Tax
has been reported to bind to the basal transcription factors TFIIA and TATA-binding protein (41, 42).
exon VII mutant Ets1 and Tax, as well as the chloramphenicol
acetyltransferase constructs containing the wild-type 
328/+20 PTHrP
fragment or the Ets1 or Sp1 mutant version of this promoter have been
described earlier (21, 46, 64-66). The expression plasmid for the
Gal4-Tax fusion protein was created as follows. First, an
EcoRI/BamHI DNA fragment of Tax encoding amino
acids 2-353 was subcloned from pLex-Tax (41) into the pCRII vector
(Invitrogen). Second, the Tax-pCRII plasmid was digested with
EcoRI and SacI, and the resulting Tax-containing
fragment was cloned into the multiple cloning site of plasmid pSG424
(67), allowing the Tax gene to be in frame with the Gal4
DNA-binding domain located upstream of Tax. Gal4-Tax mutants Gal4-M5,
Gal4-M6, and Gal4-M22 were produced in two steps by cloning the
AccI/XmaI fragments from M5, M6, or M22 (66), respectively, into Tax-pCRII followed by cloning the
EcoRI/SacI fragment from the M5-, M6-, or
M22-pCRII plasmid into pSG424. In a similar way, Gal4-M3 was created,
except that in the initial step, the PflMI/XmaI
fragment of wild-type Tax was replaced by the corresponding fragment of
M3 (66). The first 17 amino acids of Tax were deleted by digesting
Tax-pCRII with AccI and EcoRI, followed by blunt
ending with Klenow polymerase and ligation. The resulting Tax mutant
N18 was cloned into pSG424 as described above. Tax mutant M17/20
(V17G,Y18G,F20D) was generated in Tax-pCRII as follows. First, an
oligonucleotide (sense strand,
5
-TT GGA CAG AGT CTTCTT TTC GGA TAC CCA G
GT
GGA GAC TGT-3; and antisense strand,
5-G TAC ACA GTC TCC 
AC 
C TGG GTA TCC GAA AAG AAG ACT CTG TCC AAA CC-3 (mutated nucleotides are underlined, and the created
SalI site is shown in italics)) was inserted between the
PflMI site and the downstream BsrGI site,
followed by reinsertion of the Tax BsrGI fragment. The
mutant was then cloned into pSG424. By using polynucleotide chain
reaction and the pCRII vector as described (21), the Gal4 mutant
328/+20 PTHrP promoter was created, which contains a single
Gal4-binding site (CGGAGGACAGTACTCCG) in place of the PTHrP sequence
between nucleotides 63 and 47.
- and 3-ends, respectively, was synthesized by polymerase chain
reaction using high-fidelity polymerase Vent (New England Biolabs
Inc.). To generate an exon VII-deleted version of this fragment, the
wild-type BglII/AatII sequence was replaced by
the corresponding fragment of exon VII mutant Ets1. The fragments
were then cloned into the yeast shuttle vector pJG4-5 downstream of the
DNA sequences that encode the B42 acidic activator domain, the
influenza virus HA1 epitope, and the simian virus 40 nuclear
localization signal (41). The vector allowing the expression of
LexA-Tax has been described elsewhere (41). LexA-Tax was continuously
expressed in yeast cells, whereas expression of the Ets1-acidic
activator domain fusion protein or the acidic activator domain alone
was induced when yeast cells were grown under
galactose-dependent conditions. After 5-6 h of growth in
galactose-containing medium, cells were harvested and assayed for
-galactosidase activity as described.2 To determine
expression of the fusion proteins, yeast cells were lysed as
described,2 and extracts were analyzed by Western
blotting.
328/+20 PTHrP gene fragment unless Ets1 is cotransfected with Tax
(Fig. 1A). More important, Ets1 did not
change the level of Tax expression, nor did Tax have an effect on the
Ets1 level (Fig. 1B).
Fig. 1.
Cooperative effect of Tax and Ets1 on the
328/+20 PTHrP promoter fragment in OsA-CL cells. A,
chloramphenicol acetyltransferase (CAT) activity using
increasing amounts of Ets1 expression vector pKCR3ets1 in the absence
or presence of 8 µg of pCTax. B, Western blot analysis of
OsA-CL cells transfected with 8 µg of pKCR3ets1 and 8 µg of pCTax,
no DNA, 8 µg of pKCR3ets1, or 8 µg of pCTax. The blot was
subsequently probed with anti-Ets1 and anti-Tax followed by reprobing
with anti-Sp1 to show that the same amount of protein has been applied
in all lanes. C, schematic representation of a C-terminal
portion of Ets1 protein between amino acids 230 and 441. Hatched areas indicate sequences involved in autoinhibition (I) of Ets1. Calcium-dependent phosphorylation
sites in the exon VII domain are marked (P). D,
comparison of transactivation of the PTHrP promoter by exon VII
mutant Ets1 versus wild-type (WT) Ets1 in the
absence and presence of Tax. Eight µg of each expression plasmid was
used. Bars represent the average value of three (exon VII
mutant Ets1) or 11 (wild-type Ets1) independent experiments. E, Ets1-specific Western blot analysis of OsA-CL cells
transfected with control, exon VII mutant Ets1, or wild-type Ets1
plasmid DNA.
[View Larger Version of this Image (26K GIF file)]
exon VII mutant of Ets1 (Fig. 1, C,
D, and E) for its ability to functionally
interact with Tax. As shown in Fig. 1D, exon VII was
dispensable for the Tax/Ets1 synergism. In addition, the removal of the
exon VII domain did not increase the activity of Ets1 in the absence of
Tax. These results suggest that exon VII-regulated DNA binding (65, 68,
69) is not a limiting factor for Ets1 transactivation of the PTHrP P2
promoter and that Tax does not exert its effect on Ets1 through an
increase in its DNA binding activity.
Fig. 2.
A, schematic of the wild-type
(PTHrP-WT) and Gal4 328/+20 (PTHrP-Gal4)
promoters; B, effect of Gal4-Tax on the wild-type 328/+20
PTHrP promoter fragment and a Gal4 mutant version of this promoter.
OsA-CL cells were transfected with either 8 µg of Gal4-Tax expression
plasmid or pSG424-Tax alone or together with 3 µg of pKCR3ets1 or
with the Ets1 expression vector alone or with none of these plasmids.
Bars represent the mean value of seven independent
experiments. In two control experiments, 8 µg of pCTax was
cotransfected with the Gal4 PTHrP chloramphenicol acetyltransferase
(CAT) plasmid in place of pSG424-Tax.
[View Larger Version of this Image (15K GIF file)]
N18, N60, 60-284, and
C284). None of these Tax mutants were able to transactivate either
the Gal4 or the wild-type PTHrP promoter (Fig.
3B and data not shown). Subsequently, double
or triple mutations were introduced into the Tax gene,
focusing primarily on a stretch of N-terminal amino acids from
positions 9 to 25 (Fig. 3B). This sequence contains homology
to two adjacent C-terminal peptides in Ets1 (amino acids 411-419 and
419-426) that include the -strand S4 (amino acids 411-414) of the
DNA-binding domain and the linking region between the DNA-binding
domain and the C-terminal inhibitory helix H4 (amino acids 427-432)
(Fig. 3A). Of the mutations tested, only Gal4-M5 (D22A,C23S)
Tax showed a differential activation pattern (Fig. 3C). This
mutant was fully active on the Gal4 promoter, yet it showed a
significantly reduced ability to transactivate the wild-type promoter
in concert with Ets1. It is important to note that there were no
differences in expression levels of wild-type and Gal4-M5 Tax in the
presence or absence of Ets1 (data not shown). Therefore, it is likely
that the double mutation at positions 22 and 23 interfered specifically
with the ability of Tax to cooperate with Ets1.
Fig. 3.
A double mutation at amino acids 22 and 23 interferes with the Tax/Ets1 cooperative effect. A,
alignment of the Tax amino acid sequence between positions 9 and 28 with two sequences in the C-terminal part of Ets1 (amino acids
(aa) 411-419 and 419-426); B, schematic of the
Gal4-Tax mutants used for transfections into OsA-CL cells;
C, transfection experiments using either the wild-type PTHrP
(PTHrP-WT) (I and II) or the Gal4
PTHrP (PTHrP-Gal4) (III) chloramphenicol
acetyltransferase construct as a reporter plasmid. Eight µg of mutant
or wild-type Gal4-Tax was transfected alone (I and
III) or together with 3 µg of pKCR3ets1 (II).
Tax mutant-induced activities of the wild-type PTHrP promoter were
calculated relative to the activity of the wild-type promoter in the
presence of wild-type Gal4-Tax and Ets1 together (I and
II). Relative activities of the Gal4 PTHrP promoter resulted
from a comparison with the activity of this promoter in the presence of
Gal4-Tax alone (III).
[View Larger Version of this Image (36K GIF file)]
-galactosidase gene cassette, -galactosidase activity was measured and compared with that of a
control transfection with LexA-Tax and the acidic activator alone. As
shown in Fig. 4A, -galactosidase activity
was increased by ~10-fold when Ets1 was coexpressed with Tax. Similar
results were obtained with the exon VII splicing mutant Ets1 in
place of wild-type Ets1. As a control for these studies, when
Drosophila bicoid, human papilloma virus E6, or human
immunodeficiency virus Tat was fused to LexA and expressed together
with Ets1, no increase in -galactosidase activity was observed (data
not shown). These results suggest that Tax and Ets1 interact
specifically in the yeast two-hybrid system.
Fig. 4.
Tax binds to Ets1 in vivo.
A, for two-hybrid protein-protein binding analysis in yeast,
wild-type (WT) or M5 mutant Tax was fused to the LexA
DNA-binding domain and tested for its ability to transactivate a
LexA/-galactosidase gene reporter plasmid in the absence or presence
of a wild-type Ets1 or a exon VII mutant Ets1 fusion protein, each
containing an acidic activator domain (AD).
-Galactosidase activity was measured 5-6 h after culturing cells in
galactose-enriched medium to induce expression of the Ets1-acidic
activator domain fusion proteins or the acidic activator domain alone.
B, Western blot analysis was performed on extracts from
yeast cells incubated in galactose medium for 6 h. The expression
of LexA-wild-type Tax or LexA-M5 Tax was determined by an anti-LexA
antibody (lanes 1 and 2). The expression of
wild-type or exon VII Ets1 was determined by an Ets1-specific
antibody (lanes 3-5).
[View Larger Version of this Image (19K GIF file)]
exon
VII Ets1, no increase in -galactosidase activity was observed (Fig.
4A). As shown in Fig. 4B, LexA-M5 Tax was
expressed in levels comparable to wild-type Tax. Therefore, the double
mutation at positions 22 and 23 not only affected Tax/Ets1 synergism,
but also interfered with the ability of Tax to interact with Ets1 in
the two-hybrid system, suggesting that the cooperation with Ets1 and
Tax requires interaction between the two proteins.
Fig. 5.
Tax forms a ternary complex with Ets1 and
Sp1. Equal amounts (~5 ng) of Tax and/or Sp1 and Ets1 were mixed
and incubated with an anti-Ets1 antibody conjugate to immunoprecipitate
Ets1 and associating proteins under conditions as described under
"Materials and Methods." As a control, Tax or Sp1 was incubated
with the antibody beads in the absence of Ets1 (lanes 5 and
6). The washed precipitates were analyzed for the presence
of Tax, Ets1, and Sp1 by the Western blot technique. Bands indicated by
an asterisk represent the heavy chain of the anti-Ets1
antibody.
[View Larger Version of this Image (32K GIF file)]
Fig. 6.
Tax fails to cooperate with Ets1 to
transactivate an Sp1 mutant version of the 328/+20 PTHrP
promoter. An Sp1 and an Ets1 mutant PTHrP P2 promoter
(A) were tested for their abilities to respond to Tax (8 µg of pCTax) and/or Ets1 (8 µg of pKCR3ets1) in transient
transfection experiments (B) as described under "Material and Methods." CAT, chloramphenicol
acetyltransferase.
[View Larger Version of this Image (17K GIF file)]
B (M22, amino acids 137 and 138) or CREB (M47, amino acids 319 and 320) has also been observed (66). It is of interest to note that
Tax amino acids 22 and 23 are located within a sequence that shows
homology to a C-terminal motif of the Ets1 protein. Based on
preliminary studies, we speculate that this domain might be important
for intra- and intermolecular interactions with the N-terminal 37 amino
acids of Ets1. Consistent with this hypothesis, Ets2, which has
homology to the C but not the N terminus of Ets1, fails to cooperate
with Tax to transactivate the Ets1/Sp1-responsive HTLV-I long terminal
repeat (71-73).
Present address: Inst. für Zellbiologie, Abteilung
Molekularbiologie, Eberhard-Karls-Universitat Tübingen, 72076 Tübingen, FRG.
§
Present address: Dermatology Research, George Washington
University, 124 Ross Hall, Washington, D. C. 20037.
¶
Present address: Dept. of Biochemistry, Ajou University of
Medicine, Su Won 442-749, Korea.
To whom correspondence should be addressed: Virus Tumor
Biology Section, Lab. of Molecular Virology, Bldg. 41, Rm. B403, NCI, NIH, Bethesda, MD 20892-5005. Tel.: 301-496-6201; Fax: 301-496-4951; E-mail: bradyj{at}dce41.nci.nih.gov.
1
The abbreviations used are: HTLV-I, human T-cell
lymphotropic virus type I; PTHrP, parathyroid hormone-related protein;
CREB, cAMP-responsive element-binding protein.
2
Pise-Masison, C. A., Dittmer, J., Clemens, K. E., and Brady, J. N., (1997) Mol. Cell. Biol.
243-330 Ets1 deletion mutant and S. D. Gitlin and P. Lindholm for
baculovirus extracts.
©1997 by The American Society for Biochemistry and Molecular Biology, Inc.
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