Download Roles of the mammalian target of rapamycin

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Biochemical Society Transactions (2012) Volume 40, part 1
Roles of the mammalian target of rapamycin,
mTOR, in controlling ribosome biogenesis and
protein synthesis
Valentina Iadevaia*, Yilin Huo*, Ze Zhang*, Leonard J. Foster† and Christopher G. Proud*1
*School of Biological Sciences, Life Sciences Building, Highfield Campus, University of Southampton, Southampton SO17 1BJ, U.K., and †Department of
Biochemistry and Molecular Biology and Centre for High-Throughput Biology, 2125 East Mall, University of British Columbia, Vancouver, BC, Canada, V6T 1Z4
Abstract
mTORC1 (mammalian target of rapamycin complex 1) is controlled by diverse signals (e.g. hormones,
growth factors, nutrients and cellular energy status) and regulates a range of processes including anabolic
metabolism, cell growth and cell division. We have studied the impact of inhibiting mTOR on protein synthesis in human cells. Partial inhibition of mTORC1 by rapamycin has only a limited impact on protein
synthesis, but inhibiting mTOR kinase activity causes much greater inhibition of protein synthesis. Using
a pulsed stable-isotope-labelling technique, we show that the rapamycin and mTOR (mammalian target
of rapamycin) kinase inhibitors have differential effects on the synthesis of specific proteins. In particular,
the synthesis of proteins encoded by mRNAs that have a 5 -terminal pyrimidine tract is strongly inhibited
by mTOR kinase inhibitors. Many of these mRNAs encode ribosomal proteins. mTORC1 also promotes the
synthesis of rRNA, although the mechanisms involved remain to be clarified. We found that mTORC1 also
regulates the processing of the precursors of rRNA. mTORC1 thus co-ordinates several steps in ribosome
biogenesis.
Introduction
mTOR (mammalian target of rapamycin) is a serine/threonine protein kinase. mTOR interacts with other
proteins to form two distinct types of complex termed
mTORC (mTOR complex) 1 and 2. In the short term,
rapamycin, a macrolide antibiotic, inhibits some, but not all,
of the functions of mTORC1. In some cell types, over longer
treatment times, rapamycin can also interfere with mTORC2
[1].
mTORC1 and mTORC2 have distinct substrates, and this
specificity is likely to be conferred by the partner proteins
raptor (regulatory associated protein of mTOR) (mTORC1)
and rictor (rapamycin-insensitive companion of mTOR)
(mTORC2). For example, raptor interacts with substrates for
mTORC1 through their TOS (target of rapamycin signalling)
motifs. These substrates include S6K (ribosomal S6 kinase)
1 and 2, and the 4E-BPs [eIF (eukaryotic initiation factor)
4E-binding proteins]. Substrates for mTORC2 include PKB
(protein kinase B) (also called Akt) and a related kinase, SGK
(serum- and glucocorticoid-inducible kinase).
S6Ks are implicated in the control of cell and organismal
growth [2–4]. Control of protein synthesis makes an
Key words: mammalian target of rapamycin (mTOR), mRNA, protein synthesis, rapamycin,
ribosome biogenesis, translation factor.
Abbreviations used: eIF, eukaryotic initiation factor; 4E-BP, eIF4E-binding protein; mTOR,
mammalian target of rapamycin; mTORC, mTOR complex; mTOR-KI, mTOR kinase inhibitor;
PI3K, phosphoinositide 3-kinase; PKB, protein kinase B; Pol, RNA polymerase; raptor, regulatory
associated protein of mTOR; rictor, rapamycin-insensitive companion of mTOR; S6K, ribosomal S6
kinase; TFIIIC, transcription factor IIIC; TOP, terminal tract of pyrimidines; TOS, target of rapamycin
signalling.
1
To whom correspondence should be addressed (email [email protected]).
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Authors Journal compilation important contribution to cell growth and division. This is for
several reasons: (i) the bulk of the dry mass of cells is protein,
so increased cell growth or cell division generally requires
faster overall protein synthesis, and (ii) cell division, for
example, requires specific proteins involved in checkpoints
during the cell cycle, and their production is key to cell-cycle
progression.
Many mitogenic, anabolic or hypertrophic stimuli activate
protein synthesis. Initially (within minutes or a few hours),
this involves the activation of existing components of the
translational machinery, such as translation factors, and
mainly the enhanced translation of existing mRNAs (i.e.
improved ‘translational efficiency’). Over longer times (hours
to days), additional mRNAs will be transcribed and the cell
will generate more ribosomes, through activated ribosome
biogenesis, as well as translation factors. Thus, over these
longer times, there is an increase in the translational capacity
of the cell.
The S6Ks phosphorylate several substrates which are
linked to protein synthesis (mRNA translation) including
ribosomal protein S6, a component of the 40S ribosomal
subunit, eEF2K (eukaryotic elongation factor 2 kinase) [5],
eIF4B (which promotes the activity of an RNA helicase,
eIF4A) [6] and SKAR (S6K1 Aly/REF-like target) [7]
(which is related to proteins involved in pre-mRNA splicing
and mRNA transport). However, it remains unclear what
contribution the phosphorylation of these S6K substrates
makes to the control of mRNA translation.
The 4E-BPs interact with eIF4E, the protein that binds
the 5 -cap structure of cytoplasmic mRNAs, and prevent
Biochem. Soc. Trans. (2012) 40, 168–172; doi:10.1042/BST20110682
Signalling 2011: a Biochemical Society Centenary Celebration
Figure 1 Control of translational efficiency and capacity by
mTORC1
The Figure summarizes the main known mechanisms by which
signalling downstream of mTORC1 regulates translational capacity and
efficiency (e.g. ribosome levels). This occurs through regulation of mRNA
translation in the cytoplasm, Pol III in the nucleoplasm and Pol I in the
nucleoli (grey region). Black arrows indicate established links and grey
arrows connections that are not fully understood. Question marks denote
likely connections or unidentified linking components. See the text for
further details.
phosphorylation of S6 itself, but subsequent studies have
disproved this [11,12].
Secondly, mTORC1 promotes the transcription of rRNAs,
which is catalysed by Pol III (5S rRNA) or Pol I (the
other three rRNAs) (reviewed in [13]). Although connections
between mTORC1 and the transcriptional machinery that
makes rRNA have been described (e.g. [14–16]), it is still not
fully understood how mTORC1 controls rRNA synthesis
(Figure 1). It is interesting to note that recent work has
shown that mTORC2 associates with ribosomes [17,18], and
appears to be controlled by this interaction, suggesting that
ribosome number, for example, can control the activation of
PKB signalling.
Recently, a number of compounds that inhibit the kinase
activity of mTOR have been developed, including PP242 [19]
and AZD8055 [20]. These compounds, called mTOR-KIs
(mTOR kinase inhibitors) inhibit mTORC1 and mTORC2.
Their effects are thus expected to be distinct from those of
rapamycin, which only interferes with some functions of
mTORC1 [19,21]. We have examined the effects of mTORKIs and rapamycin on protein synthesis and the production
of rRNA.
mTOR-KIs inhibit overall protein synthesis
more strongly than rapamycin
it from engaging with the scaffold protein eIF4G to form
initiation factor complexes. mTORC1 phosphorylates 4EBP1 at several sites, resulting in its release from eIF4E,
allowing it to bind eIF4G. eIF4G also binds eIF4A, and
formation of eIF4E–eIF4G–eIF4A complexes is thought to
be especially important for efficient translation of mRNAs
that contain secondary structure within their 5 -UTRs
(untranslated regions) (Figure 1). Recent findings show that
4E-BPs link mTORC1 to the control of cell proliferation,
whereas S6Ks control cell growth [2].
mTORC1 also regulates ribosome biogenesis; it does so
in at least two main ways. First, it promotes the translation
of the mRNAs encoding all of the cytoplasmic ribosomal
proteins (and some other components of the translational
machinery, such as elongation factors [8]). The translation of
these mRNAs is regulated through a 5 -TOP (terminal tract of
pyrimidines), which suppresses their translation under basal
(e.g. serum-starved) conditions; their translation is increased,
e.g., by serum (Figure 1). This effect is inhibited by rapamycin
and more strongly by inhibitors of PI3K (phosphoinositide
3-kinase) such as LY294002 [9]. However, although the
ability of rapamycin to impair 5 -TOP mRNA translation
has been known for more than 15 years [10], it is not clear
how mTORC1 controls their translation. At one time, this
was thought to be mediated through the S6Ks and perhaps
We studied the effects of rapamycin and mTOR-KIs in HeLa
cells, using the standard approach of metabolic labelling of
newly synthesized proteins with [35 S]methionine. In serumfed cells, treatment with rapamycin for 2 or 6 h had only a
small effect on the rate of protein synthesis, inhibiting it by
10% or less. In contrast, mTOR-KIs exerted a much stronger
inhibition, typically causing a 30–40% reduction in the rate
of incorporation.
This difference could reflect either a role for mTORC2 in
the short-term control of translation or effects of mTORKIs on components downstream of mTORC1 which are
not affected by rapamycin. The effects of rapamycin and
PP242 were similar in cells lacking Sin1, and thus devoid
of mTORC2, and wild-type cells [19]. This implies that the
effects of mTOR-KIs on protein synthesis are mediated not
through mTORC2, but through mTORC1.
We therefore studied the effects of rapamycin and mTORKIs on proteins known to be regulated through mTORC1.
Rapamycin completely inhibited S6 phosphorylation, implying (i) that phosphorylation of S6 (and of other substrates for
the S6Ks) plays only a minor role, at most, in maintaining
protein synthesis rates, and (ii) that the phosphorylation
of these proteins cannot explain the differing effects of
rapamycin and mTOR-KIs on translation rates.
In contrast, rapamycin and mTOR-KIs had quite differing
effects on the association of 4E-BP1 and eIF4G with eIF4E.
Whereas the latter caused the dephosphorylation of 4E-BP1,
resulting in its increased binding to eIF4E and loss of eIF4E–
eIF4G complexes, rapamycin did not. Thus this branch of
mTORC1 signalling is ‘resistant’ to rapamycin in HeLa cells.
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Biochemical Society Transactions (2012) Volume 40, part 1
In principle, the ability of mTOR-KIs, but not rapamycin, to
block eIF4E–eIF4G binding could account for their differing
effects on overall protein synthesis. Further work is needed
to understand how mTOR-KIs inhibit ongoing protein
synthesis and also the mechanism by which rapamycin exerts
its modest inhibitory effect.
Effects of mTOR-KIs and rapamycin on the
synthesis of specific proteins
To explore the effects of rapamycin and mTOR-KIs on
the synthesis of specific proteins, we made use of a newly
developed heavy-isotope-labelling procedure which we call
pSILAC (pulsed stable-isotope labelling with amino acids
in cell culture). At the start of the study period, cells
are transferred to medium containing heavy-isotope (13 C
or 15 N)-tagged versions of arginine and lysine. In our
experiments, where appropriate, mTOR inhibitors were
added 30 min before this.
Newly synthesized proteins will contain ‘tagged’ versions
of lysine and arginine. After cell lysis and tryptic digestion of
proteins, peptides are analysed by MS, where the ‘light’ and
‘heavy’ versions of each peptide can be resolved, identified
and quantified. The proportion of any given peptide present
as the heavier form indicates the rate of accumulation of the
parent protein from which it is derived. Analysis of the data
reveals the impact of signalling inhibitors on the synthesis of
specific polypeptides.
Our results show that rapamycin and mTOR-KIs have
different effects on the synthesis of many proteins. In general,
mTOR-KIs inhibit their synthesis more strongly than does
rapamycin, in line with the effects of these compounds on
overall protein synthesis rates.
For proteins known to be encoded by 5 -TOP mRNAs,
such as ribosomal proteins, rapamycin inhibited synthesis
quite strongly (typically by 20–50%), which is roughly in
line with results from earlier studies (which mostly focused
on the association of such mRNAs with polyribosomes
rather than actual rates of synthesis of the corresponding
proteins). In contrast, mTOR-KIs exerted much stronger
inhibition, with inhibition of more than 50% being seen
for most of these proteins. Importantly, this suggests that
mTOR, probably mTORC1, but perhaps via another type of
mTORC [22], controls the synthesis of these proteins by
rapamycin-sensitive and -insensitive mechanisms. A key issue
is to identify how mTORC1 modulates the translation
of 5 -TOP mRNAs. The rapamycin-insensitive input to
mTOR regulation of 5 -TOP mRNAs may contribute to the
inhibitory effects of the PI3K inhibitors mentioned above,
since LY294002 also inhibits mTOR activity [23].
Rapamycin only rather weakly inhibited the synthesis
of most other proteins, with the mTOR-KIs slowing their
production by between 20 and 50%. Thus mTOR inhibition
has less impact on the synthesis of most proteins that
are not encoded by non-5 -TOP mRNAs. Interestingly,
within the set of proteins initially categorized as encoded
C The
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Authors Journal compilation by non-5 -TOP mRNAs are a number whose synthesis is
strongly affected by mTOR-KIs or rapamycin and which
appear (as judged from the database of transcriptional start
sites at http://dbtss.hgc.jp/) to contain 5 -terminal sequences
resembling a TOP. These may be previously unrecognized
5 -TOP mRNAs. An earlier study pointed to the existence
of many human mRNAs that contain a 5 -TOP, although,
for most of them, this feature was not ‘conserved’ in mice
[24]. Nevertheless, taken together, these data point to the
possibility that signalling through mTORC1 may, via 5 TOP-mediated translational control, promote the synthesis
of a larger number and diversity of proteins than has hitherto
been recognized. This mechanism may therefore mediate the
regulation by mTORC1 of other processes in addition to
ribosome biogenesis and translational capacity.
Control of ribosome biogenesis by mTOR
As noted above, mTORC1 is known to control the transcription of rRNA as well as the synthesis of ribosomal proteins
[13]. Rapamycin impairs the formation of transcriptioninitiation complexes involved in transcribing the 45S prerRNA genes [15]. The major rRNAs (5.8S, 18S and 28S) are
derived from a single primary transcript, the 45S pre-rRNA,
which is made by Pol (RNA polymerase) I. Rapamycin
causes the inactivation of TIF-1A, a component of the Pol I
transcriptional machinery, via changes in its phosphorylation.
Rapamycin causes the dephosphorylation of one site in TIF1A and increased phosphorylation of another one. It also
leads to accumulation of TIF-1A in the cytoplasm. It is
important to note that mTORC1 has been shown to associate
with the promoters for genes transcribed by Pol I and Pol III
[25,26]. mTORC1 (and its yeast counterpart) are thought
to phosphorylate components involved in Pol III function
directly, but it is not clear how mTORC1 controls Pol I.
Earlier work indicated that S6K1 played a role in
promoting the activity of Pol I [16]. The mechanism by which
rapamycin impairs rRNA transcription appears to involve
dephosphorylation of the C-terminal tail of UBF (upstream
binding factor), which binds directly to rDNA and interacts
with other components of the RNA Pol I machinery, in
particular SL-1 [16].
In the case of Pol III, which makes 5S rRNA,
regulation by TORC1 in yeast involves the phosphorylation
of Maf1, a repressor of Pol III [27]. Phosphorylated
Maf1 is excluded from the nucleus, thus alleviating the
inhibition of Pol I transcription. TORC1 appears to promote
the phosphorylation of Maf1 both directly (i.e. it can
phosphorylate Maf1) [28] and through the protein kinase
Sch9, a homologue of the mammalian S6Ks [29–31]. The latter
apparently mediates the nuclear exclusion of Maf1, whereas
TORC1 regulates the association of Maf1 with chromatin
[29].
Mammalian Maf1 undergoes dephosphorylation in response to inhibition of mTORC1 [26,32] and has been
reported to be a direct substrate for mTORC1 which
can phosphorylate it at several sites [32]. Knockdown
Signalling 2011: a Biochemical Society Centenary Celebration
or knockout of Maf1 rendered Pol III-mediated tRNA
transcription insensitive to inhibition of mTORC1 [26,32],
confirming its importance for the control of Pol II by mTOR.
In the case of mammalian Maf1, which does not contain an
obvious nuclear localization signal, mTORC1 mainly acts by
ensuring the exclusion of Maf1 from chromatin rather than
from the nucleus, which contrasts with the situation in yeast.
Another difference between yeast and mammalian Maf1 is
that the Sch9 phosphorylation sites in the yeast protein are
absent from the shorter mammalian orthologue. However, a
rapamycin-resistant mutant of S6K1 can partially reverse the
dephosphorylation of Maf1 caused by mTOR inhibition [26].
Kantidakis et al. [14] have confirmed that mTOR associates
with Pol III genes and have shown that it binds to TFIIIC
(transcription factor IIIC), which binds to Pol III promoters.
This mechanism for localization is likely to be mediated
through a TOS motif in the TFIIIC component TFIIIC63,
thereby allowing mTORC1 to phosphorylate Maf1 on
chromatin.
The 45S pre-rRNA must be processed, through multiple
steps, to yield the mature 5.8S, 18S and 28S rRNAs. Our
own recent results suggest that mTORC1 also regulates this
process. Treatment of cells with rapamycin causes the decay
of newly made rRNA and the accumulation of intermediates
in the rRNA-processing pathway, indicating that it blocks
specific steps in that process.
Concluding comments
Although important advances continue to be made in
understanding how mTOR controls cell function, there
remain important gaps in our understanding. The present
article has highlighted the fact that we do not understand
how it regulates steps involved in ribosome biogenesis, such
as the translation of 5 -TOP mRNAs, the activity of Pol I or
the processing of rRNA. It is also still unclear how mTORKIs impair general protein synthesis in HeLa cells, given that
this seems not to be mediated through inactivation of eIF4E
by hypophosphorylated 4E-BPs.
Funding
This work was supported by grants to C.G.P. from AstraZeneca, the
European Union and the British Heart Foundation.
References
1 Sarbassov, D.D., Ali, S.M., Sengupta, S., Sheen, J.H., Hsu, P.P., Bagley, A.F.,
Markhard, A.L. and Sabatini, D.M. (2006) Prolonged rapamycin treatment
inhibits mTORC2 assembly and Akt/PKB. Mol. Cell 22, 159–168
2 Dowling, R.J., Topisirovic, I., Alain, T., Bidinosti, M., Fonseca, B.D.,
Petroulakis, E., Wang, X., Larsson, O., Selvaraj, A., Liu, Y. et al. (2010)
mTORC1-mediated cell proliferation, but not cell growth, controlled by
the 4E-BPs. Science 328, 1172–1176
3 Montagne, J., Stewart, M.J., Stocker, H., Hafen, E., Kozma, S.C. and
Thomas, G. (1999) Drosophila S6 kinase: a regulator of cell size. Science
285, 2126–2129
4 Shima, H., Pende, M., Chen, Y., Fumagalli, S., Thomas, G. and Kozma, S.C.
(1998) Disruption of the p70S6k /p85S6k gene reveals a small mouse
phenotype and a new functional S6 kinase. EMBO J. 17, 6649–6659
5 Wang, X., Li, W., Williams, M., Terada, N., Alessi, D.R. and Proud, C.G.
(2001) Regulation of elongation factor 2 kinase by p90RSK1 and p70 S6
kinase. EMBO J. 20, 4370–4379
6 Shahbazian, D., Roux, P.P., Mieulet, V., Cohen, M.S., Raught, B., Taunton,
J., Hershey, J.W., Blenis, J., Pende, M. and Sonenberg, N. (2006) The
mTOR/PI3K and MAPK pathways converge on eIF4B to control its
phosphorylation and activity. EMBO J. 25, 2781–2791
7 Jastrzebski, K., Hannan, K.M., Tchoubrieva, E.B., Hannan, R.D. and
Pearson, R.B. (2007) Coordinate regulation of ribosome biogenesis and
function by the ribosomal protein S6 kinase, a key mediator of mTOR
function. Growth Factors 25, 209–226
8 Iadevaia, V., Caldarola, S., Tino, E., Amaldi, F. and Loreni, F. (2008) All
translation elongation factors and the e, f, and h subunits of translation
initiation factor 3 are encoded by 5 -terminal oligopyrimidine (TOP)
mRNAs. RNA 14, 1730–1736
9 Stolovich, M., Tang, H., Hornstein, E., Levy, G., Cohen, R., Bae, S.S.,
Birnbaum, M.J. and Meyuhas, O. (2002) Transduction of growth or
mitogenic signals into translational activation of TOP mRNAs is fully
reliant on the phosphatidylinositol 3-kinase-mediated pathway but
requires neither S6K1 nor rpS6 phosphorylation. Mol. Cell. Biol. 22,
8101–8113
10 Jefferies, H.B.J., Reinhard, G., Kozma, S.C. and Thomas, G. (1994)
Rapamycin selectively represses translation of the ‘polypyrimidine tract’
mRNA family. Proc. Natl. Acad. Sci. U.S.A. 91, 4441–4445
11 Pende, M., Um, S.H., Mieulet, V., Sticker, M., Goss, V.L., Mestan, J.,
Mueller, M., Fumagalli, S., Kozma, S.C. and Thomas, G. (2004)
S6K1 − / − /S6K2 − / − mice exhibit perinatal lethality and
rapamycin-sensitive 5 -terminal oligopyrimidine mRNA translation and
reveal a mitogen-activated protein kinase-dependent S6 kinase
pathway. Mol. Cell. Biol. 24, 3112–3124
12 Ruvinsky, I., Sharon, N., Lerer, T., Cohen, H., Stolovich-Rain, M., Nir, T.,
Dor, Y., Zisman, P. and Meyuhas, O. (2005) Ribosomal protein S6
phosphorylation is a determinant of cell size and glucose homeostasis.
Genes Dev. 19, 2199–2211
13 Mayer, C. and Grummt, I. (2006) Ribosome biogenesis and cell growth:
mTOR coordinates transcription by all three classes of nuclear RNA
polymerases. Oncogene 25, 6384–6391
14 Kantidakis, T., Ramsbottom, B.A., Birch, J.L., Dowding, S.N. and White, R.J.
(2010) mTOR associates with TFIIIC, is found at tRNA and 5S rRNA genes,
and targets their repressor Maf1. Proc. Natl. Acad. Sci. U.S.A. 107,
11823–11828
15 Mayer, C., Zhao, J., Yuan, X. and Grummt, I. (2004) mTOR-dependent
activation of the transcription factor TIF-IA links rRNA synthesis to
nutrient availability. Genes Dev. 18, 423–434
16 Hannan, K.M., Brandenburger, Y., Jenkins, A., Sharkey, K., Cavanaugh, A.,
Rothblum, L., Moss, T., Poortinga, G., McArthur, G.A., Pearson, R.B. and
Hannan, R.D. (2003) mTOR-dependent regulation of ribosomal gene
transcription requires S6K1 and is mediated by phosphorylation of the
carboxy-terminal activation domain of the nucleolar transcription factor
UBF. Mol. Cell. Biol. 23, 8862–8877
17 Oh, W.J., Wu, C.C., Kim, S.J., Facchinetti, V., Julien, L.A., Finlan, M., Roux,
P.P., Su, B. and Jacinto, E. (2010) mTORC2 can associate with ribosomes
to promote cotranslational phosphorylation and stability of nascent Akt
polypeptide. EMBO J. 29, 3939–3951
18 Zinzalla, V., Stracka, D., Oppliger, W. and Hall, M.N. (2011) Activation of
mTORC2 by association with the ribosome. Cell 144, 757–768
19 Feldman, M.E., Apsel, B., Uotila, A., Loewith, R., Knight, Z.A., Ruggero, D.
and Shokat, K.M. (2009) Active-site inhibitors of mTOR target
rapamycin-resistant outputs of mTORC1 and mTORC2. PLoS Biol. 7, e38
20 Chresta, C.M., Davies, B.R., Hickson, I., Harding, T., Cosulich, S., Critchlow,
S.E., Vincent, J.P., Ellston, R., Jones, D., Sini, P. et al. (2010) AZD8055 is a
potent, selective, and orally bioavailable ATP-competitive mammalian
target of rapamycin kinase inhibitor with in vitro and in vivo antitumor
activity. Cancer Res. 70, 288–298
21 Wang, X., Beugnet, A., Murakami, M., Yamanaka, S. and Proud, C.G.
(2005) Distinct signaling events downstream of mTOR cooperate to
mediate the effects of amino acids and insulin on initiation factor
4E-binding proteins Mol. Cell. Biol. 25, 2558–2572
22 Patursky-Polischuk, I., Stolovich-Rain, M., Hausner-Hanochi, M., Kasir, J.,
Cybulski, N., Avruch, J., Ruegg, M.A., Hall, M.N. and Meyuhas, O. (2009)
The TSC–mTOR pathway mediates translational activation of TOP mRNAs
by insulin largely in a raptor- or rictor-independent manner. Mol. Cell.
Biol. 29, 640–649
C The
C 2012 Biochemical Society
Authors Journal compilation 171
172
Biochemical Society Transactions (2012) Volume 40, part 1
23 Brunn, G.J., Williams, J., Sabers, C., Weiderrecht, G., Lawrence, J.C. and
Abraham, R.T. (1996) Direct inhibition of the signalling functions of the
mammalian target of rapamycin by the phosphoinositide 3-kinase
inhibitors, wortmannin and LY294002. EMBO J. 15, 5256–5267
24 Yamashita, R., Suzuki, Y., Takeuchi, N., Wakaguri, H., Ueda, T., Sugano, S.
and Nakai, K. (2008) Comprehensive detection of human terminal
oligo-pyrimidine (TOP) genes and analysis of their characteristics.
Nucleic Acids Res. 36, 3707–3715
25 Tsang, C.K., Liu, H. and Zheng, X.F. (2010) mTOR binds to the promoters
of RNA polymerase I- and III-transcribed genes. Cell Cycle 9, 953–957
26 Shor, B., Wu, J., Shakey, Q., Toral-Barza, L., Shi, C., Follettie, M. and Yu, K.
(2010) Requirement of the mTOR kinase for the regulation of Maf1
phosphorylation and control of RNA polymerase III-dependent
transcription in cancer cells. J. Biol. Chem. 285, 15380–15392
27 Wei, Y. and Zheng, X.F. (2009) TORC1 association with rDNA chromatin
as a mechanism to co-regulate Pol I and Pol III. Cell Cycle 8, 3802–3803
28 Wei, Y., Tsang, C.K. and Zheng, X.F. (2009) Mechanisms of regulation of
RNA polymerase III-dependent transcription by TORC1. EMBO J. 28,
2220–2230
C The
C 2012 Biochemical Society
Authors Journal compilation 29 Wei, Y. and Zheng, X.F. (2009) Sch9 partially mediates TORC1 signaling
to control ribosomal RNA synthesis. Cell Cycle 8, 4085–4090
30 Huber, A., Bodenmiller, B., Uotila, A., Stahl, M., Wanka, S., Gerrits, B.,
Aebersold, R. and Loewith, R. (2009) Characterization of the
rapamycin-sensitive phosphoproteome reveals that Sch9 is a central
coordinator of protein synthesis. Genes Dev. 23, 1929–1943
31 Lee, J., Moir, R.D. and Willis, I.M. (2009) Regulation of RNA polymerase
III transcription involves SCH9-dependent and SCH9-independent
branches of the target of rapamycin (TOR) pathway. J. Biol. Chem. 284,
12604–12608
32 Michels, A.A., Robitaille, A.M., Buczynski-Ruchonnet, D., Hodroj, W.,
Reina, J.H., Hall, M.N. and Hernandez, N. (2010) mTORC1 directly
phosphorylates and regulates human MAF1. Mol. Cell Biol. 30,
3749–3757
Received 4 August 2011
doi:10.1042/BST20110682