The Retinoblastoma Family of Proteins Directly Represses
Transcription in Saccharomyces cerevisiae*
Milica
Arneri
,
Ana
Traven
,
Lidija
Stare
in
i
, and
Mary
Sopta§
From the Department of Molecular Genetics, Rudjer
Bo
kovi
Institute, Bijenicka 54, Zagreb, Croatia
Received for publication, December 13, 2001
 |
ABSTRACT |
The retinoblastoma family of proteins are key
cell cycle regulatory molecules important for the differentiation of
various mammalian cell types. The retinoblastoma protein regulates
transcription of a variety of genes either by blocking the activation
domain of various activators or by active repression via recruitment to
appropriate promoters. We show here that the retinoblastoma family of
proteins functions as direct transcriptional repressors in a
heterologous yeast system when fused to the DNA binding domain of Gal4.
Mapping experiments indicate that either the A or the B domain of the
pocket region is sufficient for repression in vivo. As is
the case in mammalian cells, a phosphorylation site mutant of the
retinoblastoma protein is a stronger transcriptional repressor than the
wild type protein. We show that transcriptional repression by pRb is
dependent on CLN3 in vivo. Furthermore, the yeast histone
deacetylase components, RPD3 and SIN3, are
required for transcriptional repression.
 |
INTRODUCTION |
The retinoblastoma protein
(pRb)1 is a key regulatory
molecule important for cell cycle control and differentiation in a
number of mammalian cell types (1). p107 and p130 are two related family members, who share both sequence and functional properties with
the retinoblastoma protein (2). The three proteins show greatest
similarity within the so-called pocket domain, consisting of A and B
subdomains. The A and B subdomains have been reported to show weak
similarity to the general transcription initiation factors, TBP and
TFIIB (3, 4). Evolutionarily conserved homologs of pRb have been
identified in Caenorhabditis elegans (5),
Drosophila (6) and plants (7), while no homologs exist in yeast.
The retinoblastoma protein regulates gene expression presumably through
its interaction with transcriptional activators such as E2F1 (8-10),
MyoD (11), Elf1 (12), c-Myc (13), PU.1 (4), ATF2 (14), and UBF (15).
E2F1-responsive genes are repressed by pRb via the interaction of pRb
with the activation domain of E2F1, and this interaction is a
phosphorylation-dependent event. Phosphorylation of pRb
inhibits its ability to bind the activation domain of E2F1 and the
subsequent activation of E2F1-dependent transcription is
sufficient for cells to proceed through cell division (16, 17).
Previous studies have shown that the retinoblastoma protein can be
ectopically expressed in Saccharomyces cerevisiae and that
cyclin-dependent phosphorylation of the protein in yeast mimics that observed in mammalian cells (18).
In mammalian cells, the retinoblastoma protein bears intrinsic
transcriptional repression properties when tethered to DNA via the
heterologous Gal4 DNA binding domain, and this function is dependent on
an intact pocket region (19-22). The retinoblastoma protein has been
shown to interact with several proteins that may be relevant to its
ability to directly inhibit transcription in vivo. pRb has
been shown to interact with histone deacetylase (HDAC) and presumably
inhibits transcription by virtue of deacetylating chromatin in the
vicinity of the transcription initiation site (23-25). Chromatin
effects may also be mediated by the interaction of pRb with hSWI/SNF
(26-28). Another potential mechanism of transcriptional repression by
pRb is via its interaction with TAFII250 (29). TAFII250 possesses an
intrinsic kinase activity that is inhibited upon interaction with the
retinoblastoma protein (30). In vitro studies have shown
that Gal4-pRb actively represses transcription on chromatinized
templates but not naked DNA templates suggesting a biochemically
distinct mechanism of repression from that of E2F inhibition (31).
Since many aspects of transcriptional control are conserved from yeast
to man, we examined the transcriptional properties of the
retinoblastoma family in the yeast S. cerevisiae. Using fusions of pRb, p107, and p130 to the heterologous Gal4 DNA binding domain we studied the intrinsic transcriptional repression properties of these molecules in vivo. Our results show that the
retinoblastoma family of proteins can function as direct
transcriptional repressors in yeast with properties similar but
distinct from those observed in mammalian cells.
 |
EXPERIMENTAL PROCEDURES |
Strains and Growth Conditions--
Yeast S. cerevisiae strain GGY1::GG9 is a derivative of
GGY1 (gal4, gal80, tyr1, ade, leu2, his3, ura3) harboring a
lacZ reporter construct integrated at the URA3
locus (32). We used a marker swap protocol (33) to generate the strain
GGYtH::GG9 in which the TRP1 gene has
been eliminated by insertion of the HIS3 gene. Strains
NLY2::SS38-3 and NLY2::SS36 are derivatives of
NLY2 (gal4, gal80, ura3, his3, leu2, trp1, lys2) harboring a
lacZ reporter construct from the plasmids SS38-3 and SS36,
respectively, integrated at the URA3 locus (34). S. cerevisiae strains FT5
sin3 and FT5
rpd3
(35) are derivatives of FT5 (ura3, trp1, his3, leu2)
carrying
sin3::HIS3 and
rpd3::HIS3 alleles and were kindly provided by Dr. K. Struhl.
cln1,
cln2, and
cln3 deletions were constructed in strain W3031B
(ade2, trp1, leu2, his3, ura3) by a PCR-based gene deletion
method using the pFA6a-His3MX6 vector as described in (36). For
monitoring the activity of LexA fusions, plasmid JK1621 (2 µ,
URA3) containing the CYC1-lacZ reporter was used
(37). Plasmid-containing yeast strains were grown at 30 °C in 5 ml
of selective synthetic drop-out media (38), with glucose as a carbon
source. Yeast cultures were grown overnight to an optical density at
600 nm (A600) between 1-1.5.
Escherichia coli strain DH5
was used for the construction
and propagation of plasmid vectors.
Plasmid Constructs and Yeast Transformation--
Gal4 (1-147
aa) fusion constructs were constructed using the pAS2 vector
(CLONTECH). pAS2 containing Gal4-p130 and Gal4-p107 (39) were kindly provided by Dr. A. Yee. Plasmid vectors (pECE) containing mouse pRb or the phosphorylation mutant
p34 (40) were
obtained from Dr. Eldad Zacksenhaus. Gal4-pRb and Gal4-
p34 fusions
were constructed by ligation of a NcoI + BamHI
fragment isolated from the parental pECE vectors into NcoI + BamHI-cut pAS2. Gal4-
593-921 was constructed by deleting
the PstI fragment from Gal4-pRb in pAS2. A fragment
containing pRb aa 1-293 was generated by digesting pECE-pRb with
PvuII, filling in the ends with Klenow and then digesting
with NcoI. This 0.7-kb fragment was then ligated into pAS2
cut with NcoI + SmaI generating Gal4-
234-921. A fragment containing only aa 593-921 (B-domain and C terminus) was
cut with PstI from Gal4-pRb in pAS2, and ligated into
PstI-digested pAS2. Deletions of human pRb contained in the
vector pSK+ were obtained from Dr. Ed Harlow (41). These deletions were
isolated as EcoRV + BamHI fragments and then
ligated into SmaI + BamHI generating
Gal4-
180,303-421, Gal4-
180,622-714, Gal4-
180,773-909, respectively. Human deletion Gal4-
180,603-909 was
constructed by deleting the PstI fragment from
Gal4-
180,603-909 in pAS2.
pLexA fusion constructs were constructed into the pBTM116 vector (2 µ, TRP1). The LexA-pRb fusion was constructed by ligation of an
EcoRI + SalI fragment containing pRb (from the
Gal4-pRb construct) into EcoRI + SalI-cut
pBTM116. Plasmid Gal4-p130 was digested with NheI, and the
ends were filled in with Klenow and then digested with PstI.
The fragment containing the p130 sequence was ligated into pBTM116 cut
with SmaI + PstI, generating the LexA-p130
fusion. Yeast cells were transformed by the lithium acetate method
(42).
-Galactosidase Assays--
Specific
-galactosidase
activities were determined from yeast cultures by the method described
in Ref. 38) Each assay was performed in triplicate from at least three
independent transformants. The standard error was <15% except for
reporter SS36, for which the error was under 20%.
Protein Extracts and Western Blotting--
Protein extracts were
prepared as described previously (38) except that the breaking buffer
contained 1 M NaCl. Western blots were performed as
described previously (43). 100 µg of protein from each extract
preparation were analyzed except for constructs Gal4-
593-921,
Gal4-
180, 603-909 and Gal4-
1-592 for which we used 300, 400, and 500 µg of proteins, respectively. Proteins were resolved on a
12.5% SDS-polyacrylamide gel and transferred to a nitrocellulose
membrane. The nitrocellulose membrane was incubated with a 1:400
dilution of Gal4 DNA binding domain monoclonal antibody (Santa Cruz
Biotechnology), followed by a 1:3000 dilution of goat anti-mouse
horseradish peroxidase-conjugated secondary antibody (Bio-Rad). Signals
were detected by enhanced chemiluminescence as per the manufacturer's
protocol (Pierce).
 |
RESULTS |
We sought to determine whether the retinoblastoma
family of proteins could function as direct transcriptional repressors
in a heterologous yeast system using fusion proteins of pRb, p107, and
p130, respectively, to the heterologous Gal4 DNA binding domain (Fig.
1). We tested their transcription
function in vivo using chromosomally integrated
lacZ reporter constructs containing the yeast
GAL1 core promoter having Gal4 binding sites either upstream or downstream of Gcn4 binding sites (Fig.
2, A and B). The
data presented in Fig. 2, A and B show that all
three constructs inhibited transcription of the lacZ
reporter construct by ~50% regardless of whether the Gal4 binding
sites were upstream or downstream of the Gcn4 binding sites. In
addition, this repression was dependent on binding to the Gal4 sites as
a reporter construct lacking Gal4 binding sites was unaffected by
Gal4-pRb, Gal4-p107, or Gal4-p130 (Fig. 2C). These results
mimic those using similar fusion proteins in mammalian cells
(19-22).

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Fig. 1.
Schematic representation of Gal4 DNA binding
domain (DBD, aa 1-147) and various DBD fusion constructs to the pRb
family of proteins. Black dots represent cyclin
dependent kinase consensus phosphorylation sites that have been mutated
in the p34 construct.
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Fig. 2.
A, -galactosidase assay of the
transcriptional repression function of Gal4 DNA binding domain fusions
to the retinoblastoma family of proteins, pRb, p107 and p130, using the
lacZ reporter construct pGG9. B,
-galactosidase assay using the alternative lacZ reporter
construct, pSS38-3. C, -galactosidase assay using a
reporter construct, pSS36, without Gal4 binding sites. All data are the
result of at least three independent transformants assayed in
triplicate.
|
|
To identify the regions of pRb necessary for transcriptional repression
in yeast, we constructed in frame fusions to the Gal4 DNA binding
domain of several deletion mutants spanning both the N-terminal and
C-terminal portions of pRb (Fig. 1). We observed that deletion of the
majority of the C-terminal half of pRb eliminated transcriptional
repression in vivo (Fig. 3).
Deletion mutants Gal4-
234-921 and Gal4-
593-921 exhibited
-galactosidase activity nearly equivalent to or slightly higher than
that observed with the DNA binding domain control. Thus,
transcriptional repression was shown to reside in the C-terminal
portion of the retinoblastoma protein, which contains the pocket
domain. We then tested several smaller deletions within the latter half
of the molecule in a construct that also eliminated the N-terminal 180 amino acids. In each case, deletion of either the A domain or the B
domain individually did not appear to disrupt the ability to repress transcription from the reporter construct. We then tested fusions containing only the A domain or only the B domain and observed that
either domain was sufficient to confer transcriptional repression (Fig.
3). Interestingly, however, the A domain alone in the context of the
full N terminus was unable to repress transcription (Fig. 3). This may
have been due to the lower level of expression of this fusion (Fig.
4) or it may be that the N-terminal 180 amino acids influence the conformation or modification of the A domain in this context. All three deletion mutants tested
(Gal4-
180,303-421, Gal4-
180,622-714, Gal4-
180,773-909)
exhibited strong transcriptional repression from the reporter construct
to ~20% of the control level (Fig. 3). Indeed, these mutants were
stronger repressors than the wild type fusion construct perhaps because
in addition to being deletion mutants they might also be defective for
phosphorylation.

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Fig. 3.
-galactosidase assay using
various pRb deletion constructs fused to Gal4 DBD. All data are
the result of at least three independent transformants assayed in
triplicate.
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Fig. 4.
Western blot analysis of yeast extracts
containing Gal4 DBD alone or Gal4 fusion constructs. 100 µg of
total yeast extract are loaded per lane (left panel), except
Gal4- 593-921 (300 µg), Gal4- 180,603-909 (400 µg), and
Gal4- 592 (500 µg).
|
|
We verified by Western blot that the various constructs were indeed
expressed in yeast. We observed expression of Gal4-p130 and Gal4-pRb
(Fig. 4); however, we were unable to demonstrate expression of
Gal4-p107 (Fig. 4) and Gal4-
p34 (data not shown). The fact that we
observed transcriptional repression with these latter constructs
suggests that they are indeed expressed but that they are either
rapidly degraded or inefficiently extracted in our extract
preparations. We also observed expression of all of the pRb deletion
mutant fusions, although Gal4-
593-921 was expressed at relatively
lower levels than the other mutants (Fig. 4).
We also tested the transcriptional properties of the known pRb
phosphorylation site mutant,
p34 (40). As shown in Fig. 3 the
p34
fusion was a stronger repressor than the wild type pRb fusion
inhibiting transcription to 20% of control levels. This effect is
similar to that observed for the same construct in a mammalian system
(19). To address the issue of cyclin dependence of transcriptional
repression in vivo, we examined the repression properties of
Gal4-pRb, Gal4-
p34, and Gal4-p130 in yeast strains mutant for
individual G1 cyclins. Fig.
5A shows that in strains deleted for either CLN1 or CLN2 there is no
effect on transcriptional repression by Gal4-pRb, Gal4-
p34, or
Gal4-p130. However, in the CLN1CLN2cln3 strain background we
observed a derepression to nearly control levels with all three
fusions. These results show that CLN3 is required for
transcriptional repression in vivo.

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Fig. 5.
LexA fusions of pRb,
p34, and p130 assayed for transcriptional
repression in various genetic backgrounds. A,
-galactosidase assays in yeast strains deleted for CLN1,
CLN2, and CLN3, respectively. B,
-galactosidase assays in yeast strains deleted for RPD3
and SIN3, respectively. C, reporter construct on plasmid
pJK1621 (2 µ, URA3)
|
|
In mammalian cells transcriptional repression by pRb appears to involve
a direct interaction with histone deacetylase (HDAC1), which physically
interacts with the pocket domain of pRb (23-25). Therefore, we
analyzed the requirement for histone deacetylase in repression by
Gal4-pRb and Gal4-p130 using a yeast strain mutant for the yeast HDAC1
homolog RPD3. As shown in Fig. 5B, repression by
both Gal4-pRb and Gal4-p130 was abolished in the RPD3
deletion strain thus showing a dependence on histone deacetylase
function in vivo. Furthermore, repression by Gal4-pRb and
Gal4-p130 was also observed to be dependent on the RPD3
cofactor, SIN3 (Fig. 5B).
 |
DISCUSSION |
The retinoblastoma protein is a transcriptional regulatory
molecule whose ability to repress transcription of E2F dependent genes
is intimately connected with its ability to regulate the cell cycle.
Repression of transcription by pRb is thought to be mediated in two
ways. Firstly, the binding of pRb to the activation domain of E2F1
precludes the interaction of this domain with other factors such as TBP
or TFIIH, which are required for activation of transcription (44).
Secondly, the retinoblastoma protein may directly repress
transcription, and the interaction with E2F1 serves solely to recruit
pRb and associated factors to appropriate genes (21). We tested the
transcriptional capacity of the Rb protein family in yeast using
fusions of pRb, p107, and p130 proteins to the heterologous DNA binding
domain of yeast Gal4 and showed that all three pRb-family members
repress transcription from a chromosomally integrated lacZ
reporter construct. Thus, as is the case in mammalian cells, in yeast
the retinoblastoma family of proteins can directly repress
transcription of a given reporter gene simply by virtue of its being
tethered to an appropriate DNA binding site in the promoter region.
Repression by Gal4-pRb, Gal4-p107, and Gal4-p130 was absolutely
dependent on the presence of a Gal4 binding site in the reporter construct. Furthermore, we tested the repression effect of Gal4-pRb, Gal4-p107, and Gal4-p130 using two lacZ reporter constructs
that differed in the position of Gal4 binding sites relative to Gcn4 binding sites. We observed a similar level of repression for all fusion
proteins irrespective of whether the binding sites were placed upstream
or downstream of the Gcn4 binding sites. This data indicates that the
repression effect is unlikely to be a matter of steric hindrance but
rather an effect on activated transcription. It is possible that the
inhibition of activated transcription by Gal4-pRb is perhaps the result
of pRb binding to the activation domain of Gcn4. Such an interaction
would presumably result in the inhibition of activated transcription by
preventing the binding of factors such as the Swi/Snf complex to
the activation domain of Gcn4 (45). Alternatively, the retinoblastoma
protein may itself sequester the Swi/Snf complex (26-28) and thereby
inhibit Gcn4-dependent transcription. Previous studies have
shown that in mammalian cells LexA-pRb can inhibit transcription by
activators whose binding sites are located adjacent to the Lex binding
site in a reporter construct (20).
In an attempt to define the domains within the retinoblastoma protein
important for transcriptional repression we assayed several deletion
constructs for their ability to repress transcription from the reporter
construct. We showed that the N-terminal region cannot repress
transcription from the reporter when fused to the Gal4 DNA binding
domain. Analysis of deletions throughout the C-terminal half of the
molecule showed that either an intact A or B domain is sufficient for
transcriptional repression in vivo. This result is subtly
different from that seen in mammalian cells in which any disruption of
the pocket domain eliminates transcriptional repression. In experiments
that systematically examined the contribution of A and B domains to the
repression function of pRb in mammalian cells, it was observed that any
disruption of the B domain completely eliminated repression whereas
deletion of the A domain retained some residual ability to repress
transcription (22). Our results suggest that in yeast, the A and B
domains can function independently of each other to repress
transcription. Since the A and B domains have been shown to bear some
similarity to TBP and TFIIB (3, 4), they may inhibit transcription by
competing for factors/sites that interact with these general initiation
factors. The difference in domain requirements for repression between
yeast and mammalian data may be related to the fact that in yeast we
have used a chromosomally integrated reporter whereas in mammalian
cells transient transfection using plasmid-borne reporter genes were
assayed (19-22). Additionally, in pRb phosphopeptide analysis, yeast
cells do not show phosphorylation of two spots normally seen to be
hyperphosphorylated in mammalian cells (18). Since phosphorylation has
been shown to affect intramolecular interactions within the
retinoblastoma protein (46), it is possible that some aspects of pRb
function will be different in yeast as a result of this difference. A
recent study shows that an intact LXCXE
binding domain may not be essential to some aspects of pRb function in
yeast (47).
To address specific aspects of biological function related to the
mechanism of pRb repression in yeast we examined the role of
phosphorylation, G1 cyclins, and histone deacetylase
components in repression by pRb family members. In mammalian cells, the
hypophosphorylated pRb mutant, Gal4-
p34, is a stronger
transcriptional repressor than the corresponding wild type fusion (19).
In yeast the Gal4-
p34 fusion protein was also a stronger
transcriptional repressor in vivo than the wild type
Gal4-pRb fusion protein (Fig. 3). Our studies show that a
CLN1CLN2cln3 mutant strain showed reversal of
transcriptional repression by Gal4-pRb, Gal4-
p34 and Gal4-p130 in
yeast. No effect on repression was observed in the
CLN1cln2CLN3 or the cln1CLN2CLN3 strain
backgrounds. The results from the three strain backgrounds suggest that
Cln3-dependent phosphorylation is required for repression
in vivo. The fact that repression by Gal4-
p34 was also
dependent on CLN3 suggests that Cln3-dependent phosphorylation of sites outside of those mutated in the
p34 construct are required for repression in vivo. The
hypophosphorylated form of pRb has been shown in both yeast and
mammalian cells to have almost the same pattern of phosphopeptides
present as in the hyperphosphorylated form (18, 48). Therefore, it is
not unreasonable that Cln3-dependent phosphorylation could
be required for repression. Alternatively, Cln3-dependent
phosphorylation of a target factor could be required for repression
in vivo. In human cells, coexpression of cyclin E, cyclin B,
or cyclin D with Gal4-pRb caused a reversal of transcriptional
repression in vivo (19, 46). Analogous experiments using
cyclin knockouts in mammalian cells have not been done, and therefore
it is difficult to directly compare our data with results obtained with
cyclin overexpression experiments.
Since at least one mechanism of repression by pRb is via recruitment of
HDAC1 (23-25) we tested the requirement for the yeast HDAC1 homolog
RPD3 in repression by Gal4-pRb and Gal4-p130. Repression by
both constructs was abolished in the RPD3 deletion strain
(Fig. 5B). This data corroborate the requirement for HDAC1
observed in mammalian cells, as well as a recent report showing that
Gal4-pRb requires RPD3 for repression in yeast (47). In
addition, we observed that repression by Gal4-pRb and Gal4-p130 in
yeast was also dependent on the RPD3 cofactor,
SIN3. The data of Kennedy et al. (47) differ from
our studies in the requirement for SIN3 in yeast, and this
may be due in part to differences in strain background or the type of
assay used to assess function i.e. growth on
synthetic drop-out media-His versus
-galactosidase assays.
Our results show that the retinoblastoma family of proteins functions
as direct transcriptional repressors in vivo in S. cerevisiae. The domain requirements are subtly different from that
observed in mammalian cells in that either the A or B subdomain appears sufficient to mediate transcriptional repression. However, as is the
case in mammalian cells, hypophosphorylated versions of pRb exhibit
stronger transcriptional repression relative to the wild type protein.
We have also shown an effect of CLN3 on repression by pRb,
p34, and p130 in yeast, as well as a requirement for the histone
deacetylase components, RPD3 and SIN3. Future
studies should allow us to make use of both biochemical and genetic
approaches to further define the mechanism of direct transcriptional
repression by the retinoblastoma protein in vivo.
 |
ACKNOWLEDGEMENTS |
We thank Drs. M. Ptashne, K. Struhl, N. Lehming, S. Saha, E. Zacksenhaus, and E. Harlow for plasmids and
strains. We thank Dr. Zoran Zgaga for critically reading this manuscript.
 |
FOOTNOTES |
*
This research was supported by a grant (to M. S.) from the
Croatian Ministry of Science and Technology.The costs of publication of this
article were defrayed in part by the
payment of page charges. The article
must therefore be hereby marked
"advertisement" in accordance with 18 U.S.C. Section
1734 solely to indicate this fact.
Recipient of a graduate scholarship from the Croatian Ministry of
Science and Technology.
§
To whom correspondence should be addressed. Tel.: 385-1-456-0948l;
Fax: 385-1-456-1177; E-mail: msopta@rudjer.irb.hr.
Published, JBC Papers in Press, December 31, 2001, DOI 10.1074/jbc.M111900200
 |
ABBREVIATIONS |
The abbreviations used are:
pRb, retinoblastoma
protein;
TBP, TATA-binding protein;
TF, transcription factor;
HDAC, histone deacetylase;
aa, amino acid(s);
DBD, DNA binding domain.
 |
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