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Mol Cell Biol, June 1998, p. 3502-3508, Vol. 18, No. 6
0270-7306/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.
Optimal Activation of an Endogenous Gene by HOX11
Requires the NH2-Terminal 50 Amino Acids
Norma
Masson,
Wayne K.
Greene, and
Terence H.
Rabbitts*
MRC Laboratory of Molecular Biology,
Cambridge CB2 2QH, United Kingdom
Received 1 December 1997/Returned for modification 2 February
1998/Accepted 25 February 1998
 |
ABSTRACT |
The HOX11 homeobox gene was first identified through
studies of the t(7;10) and t(10;14) chromosomal translocations of acute T-cell leukemia. In addition, analysis of Hox11
/
mice
has demonstrated a critical role for this gene in murine spleen
development. A possible mode of in vivo function for the HOX11 protein
in these two situations is regulation of target genes following DNA
binding via the homeodomain, but little is known about how HOX11
regulates transcription in vivo. By performing transcriptional studies
in yeast and mammalian one-hybrid systems, a modular transcriptional
transactivation region at the NH2 terminus of HOX11 has
been functionally dissected from other parts of the protein. This
NH2-terminal region includes the previously identified short conserved Hep motif, which itself activates transcription in
one-hybrid assays. The importance of the NH2-terminal
region for the function of HOX11 in vivo was assayed by activating a HOX11-dependent gene in NIH 3T3 cells. Activation of this gene was
found to be dependent upon an intact homeodomain in HOX11, but maximal
activation was obtained only when the NH2-terminal 50 amino
acids of HOX11 was present, showing that this region of HOX11 is
important for in vivo transcriptional control of a chromosomal target
gene.
 |
INTRODUCTION |
Chromosomal translocations in acute
leukemias frequently involve activation of genes encoding proteins
involved in transcription (2, 18, 19). The HOX11
homeobox gene was first identified by cloning the breakpoints of
t(7;10)(q35;q24) and t(10;14)(q24;q11) chromosomal translocations,
found in 5 to 10% of patients with acute T-cell leukemia (5, 9,
13, 15). These translocations result in the HOX11
gene, which is normally found on chromosome 10, band q24, being placed
within the transcriptional control region of the T-cell receptor
gene on chromosome 14 or the T-cell receptor
gene on chromosome
7q35 (13, 29). The abnormal expression of HOX11 in T cells
as a result of these chromosomal translocations in humans is thought to
be a key step in the progression toward malignancy (5, 9, 13,
15), a view reinforced by the demonstration of oncogenic activity
of HOX11 in transplant recipients of MSCV-HOX11-transduced bone marrow
cells (10, 11).
The HOX11 gene belongs to a family of homeodomain-encoding
genes which includes the related Hox11L1 and
Hox11L2 genes first characterized in the mouse
(4). The homeodomain is conserved within this family, and,
like other homeodomain proteins, the HOX11 family has been implicated
in the regulation of cellular growth and differentiation. In mice,
Hox11 is essential for generation of the spleen (3, 21),
promoting survival of the splenic precursor cells (3), and
null mutation of the Hox11L1 gene results in myenteric
neuronal hyperplasia and megacolon (25).
Since the homeodomain is a sequence-specific DNA-binding element, most
homeodomain-containing proteins are believed to function through
trans regulation (direct or indirect) of specific target genes (16). Consistent with this, HOX11 is a nuclear protein and can bind to DNA in vitro (4, 26) and a fusion protein consisting of HOX11 fused to the GAL4 DNA-binding domain (GAL4-DBD) was
found to transactivate various promoters in both yeast and mammalian
cells (28). Three distinct domains of HOX11 were found to be
necessary for transactivation by this fusion protein (28), namely, the glycine-proline-rich region at the NH2
terminus, the glutamine-rich region at the COOH terminus, and the
homeodomain itself. However, the only data which pertain to functional
characteristics in non-artificial reporter assays are that the
NH2- and COOH-terminal regions of HOX11 appear dispensable
for transforming function (11) and that introduction of
HOX11 into NIH 3T3 cells results in transcription of a HOX11-dependent
gene (denoted Hdg-1 [8]). Therefore, we
have assessed regions of HOX11 required for optimal in vivo
transactivation of the endogenous Hdg-1 gene. In this stringent assay, we show that deletions in the HOX11 homeodomain (in
either the NH2-terminal arm or the third helix) prevent
induction of Hdg-1 by HOX11. We also show that the
NH2-terminal 50- amino-acid stretch is crucial for optimal
function of the HOX11 protein in vivo.
 |
MATERIALS AND METHODS |
Plasmids.
The yeast expression vector for the Y1 fusion
protein was constructed by PCR amplification of the HOX11
sequence corresponding to amino acids 252 to 330 and cloning into
NcoI-BamHI-digested pAS-CYH2 (a modified version
of pAS [6]). This intermediate construct contained a
unique KpnI site preceding the HOX11 sequence, and following digestion with NcoI and KpnI, a
second HOX11 PCR product corresponding to amino acids 1 to
241 was inserted to create the Y1 expression vector. Y1 protein lacks
amino acids 242 to 251 of HOX11, which are replaced by glycine and
threonine residues from the introduced KpnI site. Mutants Y2
to Y11 were made by PCR amplification with primers designed to amplify
the indicated regions of HOX11 and the Y1 expression vector
as the template. Y12 to Y17 were constructed by cloning of annealed
oligonucleotides.
Mammalian expression vectors for GAL4-DBD-HOX11 fusions were also made
by PCR amplification of the indicated regions of HOX11 and
subsequent cloning in frame into
BamHI-XbaI-digested pM1 vector (23).
Mammalian expression vectors for HOX11, HOX
N50, and HOX263C were
constructed by PCR amplification of the indicated regions of
HOX11 and cloning into the XbaI site of the
pEFBOS vector (17). HOX
N50 also contained a methionine
residue as the first amino acid preceding the HOX11 sequence. HOXMHEP
and HOX
HEP were also made by PCR amplification of HOX11
sequence with primers designed to introduce mutations into the Hep
motif (indicated in Fig. 3). pEFBOS-HOX
H3 was made by PCR with the
yeast expression vector for Y1 as the template. pEFBOS-HOXMDPA was made
as follows. A HOX11 sequence corresponding to amino acids 174 to 330 containing the mutated FPWM motif at amino acids 174 to 177 (changed to
MDPA and introducing a unique BamHI site) was PCR amplified
and cloned into the XbaI site of pEFBOS (clone
pEFBOS-ICMDPA). pEFBOS-ICMDPA contains two XbaI sites;
however, the XbaI site C terminal to the HOX11 sequence can
be blocked by dam methylation, allowing a second HOX11 PCR
product corresponding to amino acids 1 to 173 to be cloned into
XbaI-BamHI-digested pEFBOS-ICMDPA to create pEFBOS-HOXMDPA. The HOX
KN mutation was made by using
HOX11 sequences inserted at the
BamHI-XhoI sites of pBluescript KS. Digestion with XbaI and BglII removed the sequence corresponding to
the first 213 codons of HOX11. This was replaced by a
PCR-generated mutant sequence containing an internal deletion of amino
acids 198 to 204. pEFBOS-HOX
KN was made by PCR with this pBluescript KS-HOX
KN construct as template. The sequences of all the constructs were verified.
Yeast transformation and
-galactosidase assay.
Yeast
strain Hf7c (Clontech) was grown on yeast extract-peptone-dextrose
(YEPD) plates or in supplemented Sabouraud dextrose (SD) medium and
transformed by the lithium acetate method (22). Yeast cells
were assayed for
-galactosidase activity by a modified filter assay
(24).
Transient transfections and CAT assays.
Cos-7 cells were
maintained in Dulbecco's modified Eagle's medium (DMEM) containing
10% fetal calf serum. Lipofectin reagent (Gibco-BRL) was used for
transient transfections. For each 10-cm tissue culture dish, 5 µg of
the reporter plasmid pG5EC and 10 µg of each pM1-based expression
vector were used. A 1-µg portion of pVP65, expressing a fusion of
GAL4-DBD to the VP65 activation domain, was cotransfected with pG5EC as
a positive control. Chloramphenicol acetyltransferase (CAT) assays were
performed 36 h after transfection.
Extract preparation and Western blotting.
Yeast extracts
were prepared by sodium dodecyl sulfate extraction (22).
Cos-7 and NIH 3T3 cell extracts were prepared with 300 µl of lysis
buffer (10 mM HEPES [pH 7.6], 0.25 M NaCl, 0.5% Nonidet P-40, 5 mM
EDTA) per 107 cells. Extracts and molecular weight
standards (prestained protein molecular weight standards, 14,300- to
200,000-molecular-weight range; Gibco-BRL) were electrophoresed through
sodium dodecyl sulfate-15% polyacrylamide gels and transferred to a
nitrocellulose membrane (Schleicher & Schuell). The membranes were
blocked for 1 h in 5% Marvel (Premier Beverages) solution and
incubated with either a monoclonal GAL4-DBD antiserum (Santa Cruz
Biotechnology) or a polyclonal HOX11 antiserum (see below). The
membranes were incubated with horseradish peroxidase-conjugated
secondary antibodies prior to visualization of proteins with enhanced
chemiluminescence (ECL) detection reagents (Amersham Life Science).
The HOX11 antiserum was made from bacterially synthesized HOX11
protein. The
HOX11 coding sequence was cloned into the
pET-15b
bacterial expression vector (Novagen). Purified His-tagged
HOX11
protein was prepared and used for rabbit immunization.
Construction of NIH 3T3 clones.
NIH 3T3 cells were
maintained in DMEM containing 10% fetal calf serum. The cells were
cotransfected with linearized pEFBOS-based expression plasmid (10 µg)
and pMC1neopolyA selection plasmid (0.5 µg) with Lipofectin reagent.
At 24 h after transfection, selective medium (DMEM containing 10%
fetal calf serum and 0.5 mg of G418 per ml) was added to the cells.
After approximately 12 days in selective medium, G418-resistant clones
were picked and screened by Western blotting for expression of HOX11
protein or by genomic PCR for the presence of integrated pEFBOS vector (data not shown).
Northern analysis.
Northern analysis was carried out as
described previously (20) with 10 µg of total RNA per
lane. Probes were labelled by random priming (7). The
Hdg-1 cDNA probe was a 517-bp DpnII fragment of
the Hdg-1 cDNA, and the mouse ATP synthase (subunit c) cDNA
was a 246-bp DpnII fragment (8). Hybridization
levels were quantitated with a model 300A computing densitometer
(Molecular Dynamics).
 |
RESULTS |
The NH2-terminal region of HOX11 activates
transcription in yeast.
The ability of GAL4-DBD-HOX11 fusion
proteins to activate the transcription of a reporter gene was tested in
yeast cells. Various HOX11 sequences were fused to GAL4-DBD
in the yeast expression vector pAS-CYH2 (a modified pAS
[6]) and transformed into the Hf7c yeast strain
(containing a lacZ reporter gene with upstream GAL4
DNA-binding sites). Although a fusion of the GAL4-DBD with the complete
HOX11 sequence activated the lacZ reporter, the
yeast exhibited severe growth problems (data not shown). Thereafter, we
used a basic construct which contains a deletion of the third helix in
the HOX11 homeodomain. This clone resulted in a detectable fusion
protein with no obvious effect on yeast growth and which was still
capable of transcriptional activation (Fig.
1, Y1).

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FIG. 1.
Yeast one-hybrid assay of HOX11 transcriptional
activation domains. GAL4-DBD-HOX11 fusions are represented
schematically on the left. The position of the homeodomain is indicated
by the dashed vertical lines, and mutants lacking the third helix of
the homeodomain (amino acids 242 to 251) are marked by the symbol
H3. The ability of each fusion to activate the lacZ
reporter gene, as assessed by the presence of a blue color in the slot
blot -galactosidase assay, is indicated on the right. Apparent
differences in the intensity of blue coloration are ignored because
although expression of the GAL4-DBD alone and fusions Y1 to Y11 was
confirmed by Western blot analysis with a monoclonal GAL4-DBD
antiserum, the levels of protein expression varied (data not shown).
|
|
A series of mutant
HOX11 constructs were made, based on this
helix three-deletion mutant as a starting point, for delineation
of the
activation functions of HOX11 (Fig.
1). Activation of the
reporter gene
was observed for constructs Y2, Y3, Y8, Y9, and
Y10. Within these, the
minimum region of overlap which retains
the transactivation element is
the first 50 amino acids of HOX11.
Furthermore, deletion of this
50-residue stretch (clones Y4 to
Y7) destroys the activation activity
despite the retention of
a segment which can also activate
transcription, albeit weakly,
when present alone (Y11). The data
therefore indicate that at
least two independent regions of the HOX11
protein can transactivate
transcription in this yeast assay. A previous
study identified
three distinct regions of HOX11 required for optimal
transactivation
by a GAL4-DBD-HOX11 fusion (
28). These were
the glycine-proline-rich
region at the NH
2 terminus, the
homeodomain, and the glutamine-rich
region at the COOH terminus. The
differing results may reflect
our deletion of the third helix of the
homeodomain.
The first 50 amino acids of HOX11 can activate transcription in
mammalian cells.
The ability of the first 50 amino acids of HOX11
to function independently as an activation domain was confirmed in
mammalian cells. Sequences corresponding to amino acids 1 to 98 of
HOX11 or amino acids 1 to 50 were fused to the GAL4-DBD in the
mammalian expression vector pM1 (23) (to create pM1-N98 and
pM1-N50, respectively [Fig. 2]). These
plasmids were transfected into Cos-7 cells together with the pG5EC
reporter construct (23). While the parent vector pM1 did not
activate the reporter gene, transfection of either pM1-N98 or pM1-N50
resulted in similar levels of activation of the reporter (Fig. 2A).
Therefore, the first 50 amino acids of HOX11 can function independently
as a transcriptional transactivation domain in mammalian cells.

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FIG. 2.
The NH2-terminal region of HOX11 can
activate transcription in mammalian cells. (A) CAT assay of Cos-7 cells
after transfection of GAL4-DBD-HOX11 fusion plasmids. Cos-7 cells were
transfected with the pG5EC reporter construct alone or in combination
with various GAL4-DBD expression vectors: pM1 (expressing the GAL4-DBD
alone), pM1-N98 (expressing amino acids 1 to 98 of HOX11 fused to the
GAL4-DBD), pM1-N50 (expressing amino acids 1 to 50 of HOX11 fused to
the GAL4-DBD), pM1-N50 HEP (based on pM1-N50 but containing an
internal deletion of the Hep motif at amino acids 19 to 26), or pVP65
(expressing the VP65 activation domain fused to the GAL4-DBD). (B)
Western blot detection of GAL4-DBD-HOX11 fusions in Cos-7 cell
extracts. Extracts were prepared 36 h after transfection and
analyzed with a monoclonal GAL4-DBD antiserum. GAL4-DBD fusion proteins
were detected more readily than was GAL4-DBD, for unknown reasons. (C)
Chart showing the relative CAT activities of pM1-N98, pM1-N50, and
pM1-N50 HEP after normalization for protein expression levels. CAT
activity and protein levels were quantitated by densitometry. The
values are expressed as percentages, with pM1-N50 activity assigned the
value of 100%.
|
|
The HOX11 Hep sequence can facilitate transcriptional activation in
both yeast and mammalian cells.
Examination of the amino acid
sequence of the HOX11 NH2-terminal region for motifs which
might be important in transcription did not reveal anything other than
the previously noted Hep motif (9). The Hep sequence is
found in several homeodomain proteins and is related to the octapeptide
sequence found in many Pax proteins (1). An assessment of
its role in transactivation by HOX11 was made by preparing an internal
deletion of the Hep sequence in the expression clone pM1-N50 for
transfection of Cos-7 cells. This analysis (Fig. 2A) showed that the
deletion mutant (pM1-N50
HEP) was significantly reduced (10-fold
[Fig. 2C]) in its ability to activate transcription of the
cotransfected reporter plasmid. This diminution of function apparently
occurs as a direct result of the Hep deletion and is not an effect of
protein expression levels (as judged by CAT activities normalized to
detectable protein levels [Fig. 2B; a quantitation is given in Fig.
2C]).
The isolated Hep sequence was then tested for the ability to mediate
activation. A construct was made which encoded 15 amino
acids
encompassing the Hep motif, fused to the GAL4-DBD. This
construct was
tested in the yeast one-hybrid assay system and
found to be sufficient
for activation (Y12) (Fig.
3, upper
panels),
as was the core 8-amino-acid Hep sequence fused to the
GAL4-DBD
(Y13) (Fig.
3A, upper panel). The ability to activate reporter
gene transcription was impaired when the three-core hydrophobic
residues were mutated to glycine (Y14) (Fig.
3A, upper panel)
or when
two isoleucines were changed to valines and the leucine
was changed to
isoleucine (Y15) (Fig.
3A, upper panel). These
varied activation
effects were not due to reduced protein expression
(Fig.
3A, lower
panel).

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FIG. 3.
The Hep sequence can function as a sequence-specific
activation domain. (A) The upper panel shows a yeast -galactosidase
slot blot assay of GAL4-DBD-Hep fusions. Fusions Y10 and Y12 to Y15 are
represented schematically. The GAL4-DBD is fused to amino acids 1 to 50 of HOX11 (Y10), amino acids 16 to 30 of HOX11 (Y12), and amino acids 19 to 26 of HOX11 (Y13). Y14 and Y15 are based on Y12 but contain the
amino acid substitutions shown. After incubation of the
-galactosidase assay mixture for 2 h, the ability of each
fusion to activate the lacZ reporter gene was assessed by
the blue color as indicated (when exposed for 12 h, Y14 and Y15
yielded blue coloration comparable to the 2-h exposure of Y12 and Y13).
The lower panel shows Western blot detection of GAL4-DBD-HOX11 fusions
in yeast extracts with a monoclonal GAL4-DBD antiserum. (B) The ability
to activate in yeast is a conserved function of the Hep/octapeptide
motif. The upper panel shows the results of a yeast -galactosidase
assay of GAL4-DBD-Hep/octapeptide fusions. The fusions Y12, Y16, and
Y17 are represented schematically. GAL4-DBD is fused to a 15-amino-acid
peptide encompassing the HOX11 Hep motif (Y12), the Hlx Hep motif
(Y16), or the octapeptide sequence from Pax-2 (Y17). The lower panel
shows Western blot detection of GAL4-DBD-Hep/octapeptide fusions in
yeast extracts with a monoclonal GAL4-DBD antiserum.
|
|
The Hep sequence has been detected in several homeodomain proteins,
and the so-called octapeptide sequences of Pax proteins
are related to
the Hep sequence (
1). Although no specific function
has been
attributed to the Hep segments, our data on the ability
of the HOX11
Hep motif to assist transactivation suggests that
protein interactions
with this segment may constitute one of the
functions. As a means of
testing a role of related Hep/octapeptide
motifs in transcriptional
activation, the Hep motif from the homeodomain
protein Hlx and the
octapeptide sequence from Pax-2 were fused
to GAL4-DBD (Y16 and Y17).
Figure
3B (upper panel) shows that
both the Hlx and Pax2 sequences
behave similarly to the HOX11
Hep motif in activating transcription in
yeast at efficiencies
comparable to that of the HOX11 Hep motif when
protein levels
are compared (Fig.
3B, lower panel).
Maximal activation of the endogenous Hdg-1 gene by
HOX11 in NIH 3T3 cells requires the NH2-terminal 50 amino
acids.
The one-hybrid, fusion protein approach used above is
artificial in both the context of the mutants used (i.e., fusion with the GAL4-DBD) and the transcription-responsive assay used (i.e., a
transient-reporter assay). A more physiologically relevant system would
be the test of mutagenesis on HOX11 functions in vivo, via activation
of a gene known to be regulated by HOX11. Such a gene has recently been
identified (denoted Hdg-1) by cDNA representational difference analysis (8). Hdg-1 is upregulated in
NIH 3T3 cells stably expressing HOX11 protein, and this provides an in
vivo assay for HOX11 functional domains. NIH 3T3 cells were stably transfected with expression vectors coding for normal HOX11 proteins or
mutant forms. NIH 3T3 clones which expressed mutant HOX11 proteins were
isolated (Fig.
4A),
and protein expression was confirmed by Western blotting with a
polyclonal anti-HOX11 antiserum (Fig. 4B and C, upper panels).

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FIG. 4.
The NH2-terminal 50 amino acids of HOX11 is
required for optimal induction of Hdg-1 expression in NIH
3T3 cells. (A) Schematic representation of HOX11 wild-type and mutant
proteins expressed from the indicated HOX11 constructs made in the
pEFBOS vector. The positions of the Hep sequence, the conserved FPWM
motif, and the homeodomain are indicated in the diagram of HOX11, as
are the MDPA mutation in HOXMDPA and the mutation of the Hep sequence
in HOXMHEP. The various constructs were cotransfected into NIH 3T3
cells with a neomycin resistance vector, and individual clones were
picked. (B) The upper panel shows Western blot detection of HOX11
wild-type and mutant proteins in extracts from a representative set of
NIH 3T3 clones with a polyclonal HOX11 antiserum. The lower panel shows
Northern blot detection of the Hdg-1 transcript in RNA from
a representative set of NIH 3T3 clones expressing wild-type and mutant
HOX11 proteins. The 2.1-kb Hdg-1 transcript is detected in a
clone expressing wild-type HOX11 but not in a clone selected to contain
the empty expression vector (pEFBOS) or in normal NIH 3T3 cells. The
effect of various HOX11 mutations (as represented in panel A) on the
ability to induce Hdg-1 expression is shown. Although
expressed at a level comparable to that of HOX11, HOX N50 exhibits a
fourfold reduction in its ability to induce Hdg-1 (based on
densitometric analysis, as shown in panel D). Hybridization with an ATP
synthase probe was used as a control for loading of the Northern
filter. (C) The upper panel shows Western blot detection of HOX11 Hep
mutants in extracts from NIH 3T3 clones with a polyclonal HOX11
antiserum. Six independent clones expressing HOX HEP were obtained
and analyzed. Three clones expressing HOXMHEP are shown. The lower
panel shows Northern blot detection of the Hdg-1 transcript
in RNA from NIH 3T3 clones expressing HOX11 Hep mutants. The 2.1-kb
Hdg-1 transcript is detected in all three HOXMHEP clones but
at levels approximately 2.5-fold lower than in HOX11 wild-type clones
(based on densitometric analysis, as shown in panel D). The
Hdg-1 transcript is detected in two of the six HOX HEP
clones. Hybridization with an ATP synthase probe was used as a loading
control for the Northern filter. (D) Histogram showing relative levels
of Hdg-1 transcript in NIH 3T3 clones expressing wild-type
and mutant HOX11 proteins. In each case, three independent clones were
analyzed (except for the HOX KN mutant, which is based on data
obtained with two independent clones). The relative signal values were
obtained by densitometric scanning of autoradiographs and normalized
for levels of HOX11 transcript.
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|
Expression of
Hdg-1 was detected as a 2.1-kb transcript in
Northern analysis of RNA from NIH 3T3 clones expressing wild-type
HOX11
but not in NIH 3T3 cells or NIH 3T3 cells transfected with
only the
expression vector (Fig.
4B and C, lower panels). It was
found that
mutations of the homeodomain affected
Hdg-1 gene
activation.
Deletion of the third helix of the homeodomain prevented
Hdg-1 activation (HOX

H3) (Fig.
4B and C, lower panels,
and Fig.
4D),
while deletion of the NH
2-terminal arm of the
homeodomain (also
required for contacting DNA [
16])
resulted in low levels of
Hdg-1 expression (HOX

KN) (Fig.
4B and C, lower panels, and Fig.
4D). The effects seen with these
mutants indicate that an intact
homeodomain is required by HOX11 to
induce
Hdg-1 expression, most
probably therefore acting
through DNA binding (although not necessarily
binding to the
Hdg-1 gene itself).
In relation to the NH
2-terminal region of HOX11, which we
found to be a modular activation domain, a deletion of the
NH
2-terminal
50 amino acids of HOX11 (HOX

N50) (Fig.
4A)
was found to impair
Hdg-1 induction. Although HOX

N50
and HOX11 protein levels were
comparable (Fig.
4B and C, upper panels),
HOX

N50 induced lower
levels of
Hdg-1 RNA than did
wild-type HOX11 (Fig.
4B and C, lower
panels). Densitometric
quantitation of induced RNA levels show
an approximately fourfold
reduction of
Hdg-1 with HOX

N50 (Fig.
4D). The
NH
2-terminal 50 amino acids of HOX11 therefore appears
to
be important for the optimal induction of
Hdg-1 in NIH 3T3
cells, and residual levels of transcription observed after HOX

N50
expression are presumably mediated by other regions of the HOX11
protein.
The role of the Hep sequence in induction of
Hdg-1
expression was also assessed in NIH 3T3 clones expressing two different
Hep mutants (HOXMHEP and HOX

HEP [Fig.
4A]). HOXMHEP contains
three
amino acid substitutions in the HOX11 Hep sequence (a mutation
which
impairs the ability of the isolated Hep sequence to activate
in yeast
[Y14] [Fig.
3A]), and this protein induces
Hdg-1
expression
at levels 2.5-fold lower than HOX11 (Fig.
4C, lower panel,
and
Fig.
4D). However, the second Hep mutant, HOX

HEP (containing
an
internal deletion of the Hep motif) produced clonal differences.
Although HOX

HEP was present (as judged by Western analysis [Fig.
4C, upper panel]), some NIH 3T3 clones of HOX

HEP expressed levels
of
Hdg-1 RNA comparable to levels induced by wild-type HOX11
(HOX

HEP
clones 4 and 5 [Fig.
4C]), while others did not appear to
express
Hdg-1 at all (HOX

HEP clones 1, 2, 3, and 6 [Fig.
4C]). No clonal
variation was seen for any other HOX11 protein
studied. It is
possible that for HOX

HEP, conditions of growth or
specific cellular
environment will explain this variation, and this is
worthy of
a separate investigation. However, the present data indicate
that
the Hep sequence plays a role in the transcriptional activation
of
Hdg-1 mediated by the NH
2-terminal region of
HOX11 in vivo.
Cooperative interactions between homeodomain proteins and Pbx/exd
proteins appear to be mediated, at least in part, by a motif,
FPWM,
just upstream of the homeodomain (
16). However, mutation
of
this motif at amino acids 174 to 177 (HOXMDPA [Fig.
4A]) had
no
effect on the ability to induce
Hdg-1 expression (Fig.
4B,
lower panel, and Fig.
4D). Therefore, if HOX11 does require interaction
with a cofactor for
Hdg-1 induction, other regions of the
protein
must be involved. In addition, the COOH-terminal region of
HOX11,
which was previously thought to contribute to the HOX11
activation
potential (
28), does not appear to be required
for
Hdg-1 induction.
A deletion of 67 amino acids from the
COOH terminus of HOX11 (HOX263C
[Fig.
4A]) had no effect on
Hdg-1 induction (Fig.
4B, lower panel,
and Fig.
4D).
 |
DISCUSSION |
NH2-terminal activation domain and the Hep
sequence.
HOX11 is assumed to be an important transcriptional
regulator, both in the context of genes required for spleen development in the embryo and following deregulation of the HOX11 gene
by chromosomal translocations, for genes involved in the development of
T-lineage tumors. It is therefore important to identify regions of
HOX11 required for activation of HOX11-dependent genes in vivo. The
results presented here show that the HOX11 protein can activate endogenous gene expression (directly or indirectly) in a
homeodomain-dependent manner and that an activation domain at the
NH2 terminus of the protein is important for this.
The NH
2-terminal activation domain was initially delineated
by the yeast one-hybrid assay. All fusion proteins containing
the
NH
2-terminal 50 amino acids of HOX11 activated, including
a
fusion of amino acids 1 to 211 of HOX11 (data not shown), analogous
to
the fusion clone used in a two-hybrid assay (
12). This
mapping
of an activation domain to the NH
2-terminal 50 amino acids of
HOX11 differs from the results of a previous study which
identified
multiple regions of HOX11 involved in transactivation,
including
an NH
2-terminal glycine-proline-rich region of
about 190 amino
acids (
28). In our yeast one-hybrid
analysis, the glycine-proline-rich
region does not appear to be
important, with activation function
being localized discretely to the
NH
2-terminal 50 amino acids.
Within the NH
2-terminal 50 amino acids of HOX11 is a
conserved Hep sequence for which no function has yet been assigned. Our
data support a possible role for the Hep sequence in transcriptional
activation. First, it appears to be a necessary component of the
NH
2-terminal activation region, and second, the Hep
sequence itself
is sufficient to support activation when used in
isolation in
one-hybrid assays, with contributions from conserved
hydrophobic
residues in the core sequence. In addition, the Hep motif
of Hlx
and the related octapeptide motif of Pax-2 are both capable of
activation when fused to the GAL4-DBD. Previously, the Pax-2
octapeptide
had been shown to act in
cis as a
transcriptional repressor domain
(
14). The Pax-2 octapeptide
therefore appears to have the capacity
for modular transcriptional
function, being capable of repression
when studied in the context of
the Pax-2 protein (
14) and activation
when studied in
isolation. The Hep sequence may also act in both
positive and negative
transcriptional regulation.
Regions of HOX11 required for Hdg-1 induction.
It
has become apparent that protein activation domains mapped in reporter
assays may not necessarily be important for all in vivo function; e.g.,
studies of the GATA-1 transcription factor have shown that an
obligatory activation domain mapped by standard reporter assays is
dispensable for GATA-1 function in terminal erythroid cell
differentiation (27). The relevance of transcriptional assays which identify protein segments with activation potential in
isolation from the intact native protein is therefore at issue. Although the NH2-terminal 50 amino acids of HOX11 could
function as an activation domain in one-hybrid assays, it was therefore important to evaluate the role of this region in a functional assay
that provides a stringent analysis of HOX11 protein function. In
particular, such an assay could provide insight into the differing results obtained from one-hybrid analyses of HOX11 (reference 28 and data presented above). The gene
Hdg-1 is activated after HOX11 expression in NIH 3T3 cells
(8), and this chromosomal activation provides a powerful
physiological model for analysis of HOX11 functional domains. Whether
the induction of Hdg-1 expression is a direct effect of
HOX11 on the Hdg-1 promoter is unknown. Mapping of the
Hdg-1 promoter to identify the "HOX11-responsive element" is in progress to clarify this (8a).
The induction of
Hdg-1 expression in transfected NIH 3T3
cells was found to be dependent on the intact homeodomain. When the
NH
2-terminal 50 amino acids of HOX11 was deleted,
Hdg-1 induction
was significantly impaired (fourfold), in
agreement with mapping
of this region as an activation domain in
one-hybrid reporter
assays. The NH
2-terminal 50 amino acids
of HOX11 is therefore
important for in vivo transcriptional control of
a chromosomal
gene. The Hep sequence, located within the
NH
2-terminal 50 amino
acids of HOX11, may play a role in
this activation of
Hdg-1 transcription.
However, since
regions of HOX11 other than the NH
2 terminus contribute
to
Hdg-1 induction, the in vivo role of the Hep sequence is
difficult
to assess. If the Hep sequence is an important component of
the
NH
2-terminal region, one would expect Hep mutants to
exhibit a
reduced level of
Hdg-1 induction comparable to
that of the HOX

N50
mutant. Consistent with this, the HOXMHEP mutant
(containing three
amino acid substitutions within the conserved Hep
sequence) activates
Hdg-1, with the induced level close to
that of HOX

N50 (Fig.
4D).
Although the NH
2-terminal 50 amino acids is important for
Hdg-1 induction, other regions of the HOX11 protein are
clearly
involved. The homeodomain is essential, since deletions within
this domain (either of the third helix or of the
NH
2-terminal
arm) result in the loss of
Hdg-1
induction. Both these regions
of the homeodomain are predicted to
contact DNA, and so deletions
made presumably result in a loss of HOX11
DNA-binding activity.
The HOX11 homeodomain has also been suggested to
contain activation
function (
28) and may therefore be
playing a dual role in
Hdg-1 induction. In our yeast
one-hybrid analysis, we were unable to
study the intact HOX11
homeodomain due to growth retardation in
the presence of the functional
homeodomain, and it is therefore
impossible for us to address this
question unless specific mutations
which ablate the activation function
of the homeodomain while
retaining DNA-binding activity can be
identified. However, the
COOH-terminal glutamine-rich region of HOX11,
which previously
had been thought to contain activation function
(
28), did not
activate in our yeast one-hybrid analysis and
also does not appear
to be required for
Hdg-1 induction.
Mechanism of Hdg-1 induction.
The importance of
the NH2-terminal 50 amino acids of HOX11 for activation of
a chromosomal gene in vivo may provide insights into the mechanisms of
HOX11-mediated transcriptional regulation by helping to identify
interacting factors. Apart from the conserved Hep sequence, the
NH2-terminal 50 amino acids does not resemble any known
transcriptional regulatory domain. However, since this region is
involved in Hdg-1 induction (but not necessarily by direct
binding to the Hdg-1 promoter), it presumably interacts, either directly or indirectly, with a component(s) of the basal transcriptional machinery. The identification of such factors should
further delineate the role of the conserved Hep sequence and help to
elucidate the functional role of HOX11 in both splenogenesis and
tumorigenesis.
 |
ACKNOWLEDGMENTS |
N.M. was supported by an LRF fellowship, and W.G. was supported
by a C. J. Martin fellowship.
We thank A. Forster for important technical help throughout this
project.
 |
FOOTNOTES |
*
Corresponding author. Mailing address: MRC Laboratory
of Molecular Biology, Hills Rd., Cambridge CB2 2QH, United Kingdom. Phone: 1223-402286. Fax: 1223-412178. E-mail:
thr{at}mrc-lmb.cam.ac.uk.
 |
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Mol Cell Biol, June 1998, p. 3502-3508, Vol. 18, No. 6
0270-7306/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.
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