MINIREVIEW
Regulation of mammalian translation factors by nutrients
Christopher G. Proud
Division of Molecular Physiology, School of Life Sciences, University of Dundee, MSI/WTB Complex, Dow Street, UK
Protein synthesis requires both amino acids, as precursors,
and a substantial amount of metabolic energy. It is well
established that starvation or lack of nutrients impairs pro-
tein synthesis in mammalian cells and tissues. Branched
chain amino acids are particularly effective in promoting
protein synthesis. Recent work has revealed important new
information about the mechanisms involved in these effects.
A number of components of the translational machinery are
regulated through signalling events that require the mam-
malian target of rapamycin, mTOR. These include transla-
tional repressor proteins (eukaryotic initiation factor
4E-binding proteins, 4E-BPs) and protein kinases that act
upon the small ribosomal subunit (S6 kinases). Amino acids,
especially leucine, positively regulate mTOR signalling
thereby relieving inhibition of translation by 4E-BPs and
activating the S6 kinases, which can also regulate translation
elongation. However, the molecular mechanisms by which
amino acids modulate mTOR signalling remain unclear.
Protein synthesis requires a high proportion of the cell’s
metabolic energy, and recent work has revealed that meta-
bolic energy, or fuels such as glucose, also regulate targets of
the mTOR pathway. Amino acids and glucose modulate a
further important regulatory step in translation initiation,
the activity of the guanine nucleotide-exchange factor
eIF2B. eIF2B controls the recruitment of the initiator
methionyl-tRNA to the ribosome and is activated by insulin.
However, in the absence of glucose or amino acids, insulin

reviews our current understanding of the regulation of
translation factors by nutrients and recent studies applying
this information to tissues such as pancreatic b-cells, skeletal
muscle and heart. Early data suggested that, in muscle
in vivo, the rate of elongation may limit protein synthesis
under fed conditions [1] while initiation may be limiting in
starved animals [2]. Recent work has improved our
understanding of the molecular mechanisms involved in
regulating both translation initiation and elongation.
Early studies focused on the control of protein synthesis
in skeletal muscle, as it is a tissue of particular importance
for whole body protein metabolism. Overnight fasting led to
the disaggregation of polyribosomes in rat skeletal muscle
[1] indicating an impairment of translation initiation.
Fasting of animals for longer periods involved an additional
reduction in the levels of ribosomes in the tissue, manifested
as a fall in its RNA content (the bulk of cellular RNA is
ribosomal RNA) [3]. In this article I shall discuss mecha-
nisms by which nutrients regulate both translation initiation
and ribosome biogenesis.
REGULATION OF eIF4E
BY eIF4E-BINDING PROTEINS
The eukaryotic initiation factor (eIF) 4E binds to the 5¢-cap
structure of eukaryotic mRNAs and likely provides the first
contact between the translational machinery and the
mRNA in de novo translation initiation. eIF4E also interacts
with several types of protein binding partners. One class
comprises the scaffold proteins of the eIF4G group (eIF4G
I
Correspondence to C. G. Proud, Division of Molecular Physiology,

interact with the same, or overlapping, sites in eIF4E [6,8].
Binding is therefore mutually exclusive and, for example,
eIF4E bound to 4E-BP1 cannot interact with eIF4G to
form initiation complexes. 4E-BP1 thus acts as a repressor
of cap-dependent translation [9,10].
Of the three 4E-BPs, 4E-BP1 is easily the most intensively
studied and best understood. It undergoes phosphorylation
at multiple sites in vivo.AsindicatedinFig.1B,thesesites
are located almost throughout its short sequence of around
118 amino acids, only the N-terminus being devoid of sites
of phosphorylation. Phosphorylation of 4E-BP1 shows a
marked hierarchy in vivo [11,12](X.Wang,W.Li,J.L.Parra,
A. Beugent & C.G. Proud, unpublished observations).
Phosphorylation of the threonines near the N-terminus is
required for modification of Thr70, while phosphorylation
at Thr70 is required for phosphorylation of Ser65. Earlier
data suggested that Ser65 and Thr70 were the most
important sites for modulating the binding of 4E-BP1 to
eIF4E – phosphorylation at Thr70 promotes its release and
phosphorylation at Ser65 may prevent rebinding (Fig. 1B).
Phosphorylation of Ser112, at the extreme C-terminus, also
appears to be required for release of 4E-BP1 from eIF4E
([14]; C. G. Proud, unpublished data).
Phosphorylation of several sites in 4E-BP1 is increased by
agents that activate protein synthesis, such as insulin.
Phosphorylation of Ser65 and Thr70, and to a lesser extent,
Thr37/46, is blocked by rapamycin, indicating an essential
role for mTOR in signalling from, e.g., the insulin-receptor
to 4E-BP1 (reviewed in [15]). The complex nature of the
hierarchy of phosphorylation of the other sites in 4E-BP1

binding motif for interaction with eIF4E, and two regulatory domains
have also been identified – the RAIP motif towards the N-terminus
[110] and the TOS motif at the extreme C-terminus [111]. Six sites of
phosphorylation have been identified. All except S112 (numbering
based on human sequence) are Ser-Pro or Thr-Pro sites (Ser112 is fol-
lowed by Gln). Inhibition of mTOR or amino acid withdrawal results in
dephosphorylation of a number of sites in 4E-BP1, especially Ser65 and
Thr70 (underlined), although Thr37/46 are also affected. Insulin
stimulates phosphorylation of Ser65, Thr70 and Ser112, while Ser83
appear to be basally phosphorylated. Thin arrows indicate interplay
between sites of phosphorylation that underlies the complex hierarchy
of phosphorylation events, while thick arrows indicate the roles of
specific sites in regulating the function of 4E-BP1. For example, phos-
phorylation at Thr37/46 is required for phosphorylation at Thr70, and
phosphorylation at Thr70 is required for phosphorylation at Ser65.
Ó FEBS 2002 Control of translation factors by nutrients (Eur. J. Biochem. 269) 5339
hamster ovary cells are transferred to a medium lacking
amino acids, 4E-BP1 undergoes dephosphorylation [18–20],
which occurs within 15–30 min after amino acid withdrawal.
In control cells, in medium containing amino acids, little or
no 4E-BP1 is bound to eIF4E and high levels of eIF4F
complexes are present. Removal of amino acids quickly
causes a marked increase in the amount of 4E-BP1 associated
with eIF4E and loss of eIF4F complexes [18,19,21]. These
effects are similar to those of adding rapamycin suggesting
that the effects of amino acids are mediated via the mTOR
pathway. They are reversed, within minutes, by the readdi-
tion of amino acids. The most effective single amino acid is
leucine, others having very little or no effect in most cells. This
role for leucine is a feature that

4E-BP1 in the absence of any external amino acids [24,25].
Insulin elicits an increase in 4E-BP1 phosphorylation in
amino acid-replete cells, and it can still do so to some extent
in CHO cells deprived of amino acids, provided that a
metabolizable glucose analogue (or other metabolizable
hexose such as
D
-mannose) is also present [19]. Glucose
increases the basal level of phosphorylation of Thr70, but
has little effect on basal phosphorylation at Ser65 or Thr37/
46. However, the presence of glucose does allow insulin to
elicit phosphorylation at these sites [19]. Glucose thus exerts
a permissive effect with respect to the action of insulin and
promotes the release of 4E-BP1 from eIF4E to allow
formation of eIF4F complexes. This may reflect an input
from metabolic energy to the control of 4E-BP1, perhaps
via modulation of the activity of mTOR [26]. This will be
discussed in more detail below.
Overall, these data indicate a requirement both for amino
acids (especially leucine) and an energy source for activation
of this key step in translation initiation. This clearly makes
excellent sense – amino acids are the precursor for protein
synthesis, leucine being an essential amino acid, and protein
synthesis consumes a large proportion (perhaps 20–25%
[27]) of total cellular energy.
REGULATION BY AMINO ACIDS IN
PRIMARY TISSUES AND CELL TYPES
While many groups have studied the control of translation
factors in established cell lines, Kimball, Jefferson and
colleagues have extensively investigated the effects of amino

proteins, yielding a total of four S6 kinases. The activation
of S6K1 and S6K2 by all stimuli so far tested (e.g. insulin,
growth factors and phorbol esters) is blocked by rapamycin
[40,42,43].
Activation of the S6Ks involves their phosphorylation at
multiple sites, some of which lie in the catalytic domain or its
Fig. 2. The S6 kinases. The structures of S6K1 and S6K2 are depicted
schematically, including their splice variants (forms I and II is each
case). The domains within each sequence are indicated, as are major
sites of phosphorylation that are associated with activation of these
enzymes, and the nuclear localization signals (NLS) in S6K1 I and the
S6K2 isoforms. See text for further information.
5340 C. G. Proud (Eur. J. Biochem. 269) Ó FEBS 2002
so-called ÔextensionÕ or ÔlinkerÕ while the majority are located
in the C-terminal regulatory domain [40,41] (Fig. 2). Thr229
in the T-loop of the catalytic domain has been shown to be
phosphorylated by phosphoinositide-dependent kinase 1
(PDK1) in vitro [44,45]. The protein kinases responsible for
phosphorylating the other sites await conclusive identifica-
tion. While mTOR can phosphorylate T389 in vitro, it is not
clear that it is the physiological T389 kinase (discussed in
[40]). The sensitivity of S6K regulation to rapamycin
nevertheless shows that mTOR makes an essential input
to the control of the S6Ks. The interplay between the
phosphorylation sites in S6K1 is complex and for a more
detailed discussion the reader is directed to a recent
comprehensive review [40]. For the present purposes, this
can be summarized to say (a) that phosphorylation of the
sites in the C-terminus of S6K1 is believed to facilitate access
(by the relevant kinases) to Thr229 and Thr389, phos-

acids and an additional input (e.g. from insulin) reflects
effects of these agents on different (subsets of) phosphory-
lation sites in S6K1. In this context it is notable that Hara
et al. [20] reported that addition of high levels of amino
acids to CHO cells overexpressing the insulin receptor
resulted in as high a degree of activation of S6K1 as was
observed with normal levels of amino acids plus insulin.
This suggests that amino acids can elicit the full response if
present at sufficiently high levels. It seems likely that in
CHO cells, and probably in other cell types too, amino-acid-
replete cells contain only enough amino acids to give partial
activation of S6K1 and insulin provides a further input to its
activation. Some cell types appear to contain enough amino
acids for regulation of S6K1 even when starved for external
amino acids (e.g. hepatoma cells [47]). The fact that such
cells become dependent upon external amino acids when
treated with a compound that inhibits autophagy suggests
that this intracellular supply of amino acids is derived from
this form of protein breakdown. Autophagy is especially
active in hepatocytes and related cell-types, and this is
perhaps why some other cell types are more dependent on
external amino acids.
The effects on S6K1 of nutrient stimuli and agents such as
insulin are blocked by rapamycin [30]. In fact, removal of
amino acids from CHO cells leads to effects on 4E-BP1 and
S6K1, which are qualitatively similar to those of rapamycin
treatment. Similar data have been reported for a number of
other cell types including adipocytes and HEK 293 cells
[20,48], giving rise to the notion that the effects of amino
acids are transmitted via mTOR, although there is no

other targets for mTOR signalling is influenced by the size
of an intracellular pool of amino acids whose size is
determined by the rates of protein degradation and
synthesis, and by the availability of extracellular amino
acids (which presumably enter this pool following their
transport into the cell [51]).
Studies using ÔrealÕ cells, adipocytes and skeletal muscle,
generally reflect the data obtained in other, transformed, cell
lines. For example, amino acids have been shown to
stimulate S6K1 in rat adipocytes, and this effect is blocked
by rapamycin [48]. However, insulin can activate S6K1 in
isolated rat adipocytes in the absence of added amino acids
[52]. Orally administered leucine elicits the phosphorylation
of S6K1 in skeletal muscle, and this requires insulin, but not
an increase in insulin concentration [53]. In human forearm
muscle, branched-chain amino acids elicit phosphorylation
of S6K1 [54]. These data are largely similar to those
discussed above for the regulation of 4E-BP1 by amino
acids.
Ó FEBS 2002 Control of translation factors by nutrients (Eur. J. Biochem. 269) 5341
A ROLE FOR S6 PHOSPHORYLATION
IN RIBOSOME BIOGENESIS?
Work, in particular from the laboratory of George Thomas,
has suggested that the S6 kinases may play a role in
regulating the translation of a set of mRNAs termed the
5¢-TOP (tract of oligopyrimidine) mRNAs (Fig. 3). This
group of mRNAs includes those for each of the ribosomal
proteins in mammals, and those for certain other proteins
involved in mRNA translation such as the elongation
factors eEF1A and eEF2 [55] and the poly(A)-binding

S6K1) but concluded that, by various criteria, S6 phos-
phorylation did not appear to be sufficient for increased
5¢-TOP mRNA translation, at least in response to amino
acids. For example, amino acid regulation of 5¢-TOP
mRNA translation is still observed in cells in which both
alleles of the S6K1 gene are knocked out and in which no
phosphorylation of S6 is observed in response to amino
acids. This also casts doubt on the role of S6Ks and thus S6
phosphorylation in the control of 5¢-TOP mRNA transla-
tion at least in response to amino acids.
The above findings underline the need for further work to
elucidate the mechanisms by which 5¢-TOP mRNA trans-
lation is controlled and to define the cellular functions of the
S6Ks, which clearly do include roles in events linked to the
control of cell and organism size. One such function that has
recently been reported is in the control of the elongation
factor eEF2.
Elongation factor 2 (eEF2) is regulated through the
mTOR pathway and by cellular energy status eEF2
mediates the translocation step of elongation. Phosphory-
lation of eEF2 at Thr56 inhibits its activity by preventing it
from binding to the ribosome. Phosphorylation of eEF2 is
catalysed by eEF2 kinase, an unusual and highly specific
enzyme. A more detailed discussion of eEF2 and eEF2
kinase can found in the accompanying article by Browne
and Proud [65].
In CHO cells overexpressing the insulin receptor, insulin
brings about the rapid dephosphorylation of eEF2, con-
comitantly with accelerating the rate of elongation. Both
effects were blocked by rapamycin [66]. Insulin also elicits

The question mark by the role of S6 phosphorylation in the translation
of 5¢-TOP mRNAs denotes the fact that Tang et al. [64] have recently
challenged the prevailing concept that these mRNAs are regulated via
S6 kinases/phosphorylation of rpS6, at least in response to amino
acids. The question mark by the role of ATP in regulating mTOR
activity [26] is to indicate that recent data also suggest a role for the
AMP-activated kinase in regulating mTOR signalling in skeletal
muscle (see text [88]).
5342 C. G. Proud (Eur. J. Biochem. 269) Ó FEBS 2002
S6K1, which results in decreased activity of eEF2 kinase
[69]. Given these inputs from mTOR into the control of
eEF2 kinase, one might anticipate that the phosphorylation
of eEF2 would be modulated by nutrients. Indeed, insulin
cannot fully elicit the dephosphorylation of eEF2 in CHO
cells lacking amino acids or glucose [21].
A further input from the nutritional status of the cells to
the control of eEF2 phosphorylation appears to be related
to cellular ATP levels, and may underlie the requirement for
glucose referred to above. It clearly makes physiological
sense for protein synthesis to be matched to energy
availability: protein synthesis is a major energy-consuming
process, using up around 25–30% of total cellular energy
[27]. This is discussed further in the article by Browne and
Proud [65] that accompanies this one.
mTOR
Frequent mention of mTOR in the regulation of translation
has been made in this article without any discussion, so far,
of this protein itself. mTOR is large (around 290 kDa) and
its primary sequence indicates the presence of a number of
potential functional domains [71] (Fig. 4). These include a

important area for future study. One further profitable area
for investigation is likely to be the identification of proteins
that interact with mTOR; this may shed important light on
the regulation of mTOR and on downstream signalling
from mTOR to the translational machinery. Indeed, two
recent studies identified a novel 150 kDa protein termed
Raptor (regulatory associated protein of mTOR) that
interacts with mTOR [78,79]. Although raptor has a
positive role in nutrient stimulated signalling, its association
with mTOR negatively regulates mTOR kinase activity.
Nutrient withdrawal results in an increased association of
raptor with mTOR [79]. Biochemical studies show that
raptor is required for phosphorylation of 4E-BP1 by mTOR
and that raptor also enhances phosphorylation of S6k1 by
mTOR [78]. Raptor seems to have a positive role in the
regulation of cell size. It was shown to interact with 4E-BP1,
especially hypophosphorylated forms of the latter and thus
appears to act as a scaffold protein by forming ternary
complexes with mTOR and 4E-BP1 (and perhaps also
S6k1). Further work will clearly be required to determine
how nutrients modulate the mTOR–raptor interaction, and
whether raptor promotes phosphorylation of all sites in
S6k1 and 4E-BP1, or only specific residues.
Other recent work has provided insights into the
upstream regulation of mTOR. The proteins hamartin
and tuberin (also termed TSC1/2) form a complex that
suppresses signalling via mTOR (reviewed in [80]). The
genes for TSC1/2 are mutated in people suffering from
certain types of benign tumours. TSC2 is phosphorylated by
protein kinase B, this providing a potential link between

observe any effect of amino acid withdrawal on the kinase
activity of mTOR. This and several considerations lead to
doubts whether mTOR is indeed the physiological T389
Fig. 4. mTOR. The principal features of mTOR are indicated: these
include domains termed ÔtoxicÕ or ÔrepressorÕ basedonexperiments
performed with the yeast homologue TOR. FRB, FKBP12/rapamycin
binding domain. Further information is given in the text.
Ó FEBS 2002 Control of translation factors by nutrients (Eur. J. Biochem. 269) 5343
kinase [40]. Furthermore, while Thr389 appears to be the
final site in the ordered phosphorylation of S6K1, the
phosphorylation of other, ÔearlierÕ, sites is sensitive to
rapamycin (see [40] and above) strongly implying that
mTOR makes further inputs to the regulation of S6K1
additional to any effect it has on Thr389. Such inputs may
involve regulation of the (unknown) kinases acting at the
C-terminal proline-directed sites or effects on the protein
phosphatases acting on S6K1. A potential regulator of the
phosphatases acting on S6K1 is a4, which interacts with the
catalytic subunit of protein phosphatase (PP)2A [84] and is
the mammalian homologue of the yeast phosphatase
partner Tap42p [85,86], which in turn is implicated in
signalling from yeast TOR to translation. The observations
that PP2A interacts with S6K1 and is activated by
rapamycin treatment of cells might provide a mechanism
by which rapamycin causes dephosphorylation of S6K1
[40,87].
mTOR has a high K
m
for ATP for its phosphorylation of
4E-BP1 or S6K1 in vitro (around 1 m

7
and inhibition of eEF2, and is discussed
in the article by Browne and Proud [65]. Bolster et al.[88]
have recently reported that injection of a drug that activates
AMPK causes inhibition of mTOR signalling in skeletal
muscle. However, activation of AMPK may also interfere
with the synthesis and/or release of insulin [89,90], making it
hard to interpret these data. The effects on muscle mTOR
signalling may, for example, reflect changes in circulating
insulin levels.
CONTRIBUTION OF mTOR SIGNALLING
TO THE REGULATION OF PROTEIN
SYNTHESIS
IN VIVO
An important question is, to what extent do the above
regulatory events, linked to mTOR signalling, contribute to
the activation of protein synthesis in cells and tissues? This
question can be addressed by exploring the effect of
rapamycin on the control of overall rates of protein
synthesis. In cell lines, rapamycin generally exerts only a
small inhibitory effect on the rate of protein synthesis [10].
In skeletal muscle, the activation of protein synthesis
elicited by leucine is inhibited by rapamycin, but only
partially [91]. This suggests that leucine may operate to
stimulate protein synthesis both via mTOR-dependent and
-independent pathways. Indeed, leucine is still able to
activate muscle protein synthesis in alloxan-diabetic rats
where there is no S6K1 phosphorylation or eIF4E/eIF4G
binding in response to oral leucine [53]. This points to the
operation of perhaps two amino acid regulated responses:

factor that recruits the initiator methionyl-tRNA to the
40S subunit to recognize the start codon during translation
initiation [98] (Fig. 5). Binding of eIF2ÆGTPÆMet-tRNAi
complexes to the 40S subunit is therefore required for every
initiation event. The activity of eIF2B plays a role in
regulating overall and transcript specific translational con-
trol in eukaryotes from yeast to mammals, and is regulated
by a variety of inputs [98]. It can be regulated by amino
acids, apparently via several distinct mechanisms, although
these do not appear to involve signalling via mTOR. For
example, rapamycin does not affect the ability of insulin to
activate eIF2B in CHO.T cells [99]. Activation of eIF2B in
these cells requires the presence of amino acids and glucose
in the medium [21]. These effects do not appear to be
connected with defects in the ability of insulin to promote
the dephosphorylation of a regulatory (inhibitory) phos-
phorylation site at Ser535 in the e-subunit of eIF2B (which
still occurs in the absence of glucose or amino acids [21]).
In yeast, amino acids regulate the phosphorylation of the
a-subunit of eIF2, via the eIF2a kinase GCN2 (Fig. 5).
During amino acid starvation, uncharged tRNA accumu-
lates and activates Gcn2, leading to phosphorylation of
eIF2, to yield eIF2(aP), a potent inhibitor of eIF2B and
hence of translation initiation [98]. Inhibition of eIF2B leads
to increased translation of the mRNA for GCN4, an
5344 C. G. Proud (Eur. J. Biochem. 269) Ó FEBS 2002
activator of genes required for amino acid biosynthesis,
allowing yeast cells to make the necessary amino acids.
Although orthologues of Gcn2 exist in mammals, such a
mechanism does not seem to be involved in the effects of

amino acid imbalance and was probably not one of the
kinases previously shown to phosphorylate eIF2Be.This
finding further underlines the possibility that nutrient
regulation of eIF2B involves changes in its state of
phosphorylation. The activity of this unidentified kinase
was not affected by rapamycin pretreatment of the cells,
again indicating that mTOR is not involved here.
Why should mammalian cells have an additional mech-
anism to regulate eIF2B in response to amino acids given
that they also possess orthologues of GCN2? As discussed
above, in yeast, a rise in uncharged tRNA ultimately
switches on amino acid biosynthetic pathways. In contrast,
mammalian cells are unable to make many of these amino
acids, to provide substrates for tRNA charging. An
uncontrolled accumulation of uncharged tRNA could have
serious consequences for the cell by leading to misincorpo-
ration or premature termination during elongation. It may
therefore be important for mammalian cells to react to
amino acid deficiency before significant accumulation of
uncharged tRNAs occurs.
HOW DO MAMMALIAN CELLS SENSE
AMINO ACIDS?
Leucine appears to be the only amino acid capable of
eliciting an effect on 4E-BP1 and S6K1 in skeletal muscle
[35,91,101], while other branched-chain amino acids (iso-
leucine, valine) are also effective in liver [28,60]. In CHO
cells, leucine was the only one of the amino acids tested
which, when added alone, stimulated 4E-BP1 phosphory-
lation [18]. Similarly, it was omission of leucine that had the
most profound effect on the activity of S6K1 [20]. Omission

and thus also of overall translation. The importance of this mechanism
in the control of translation by amino acids in mammalian cells is so far
less clear. Recent work suggests that also that eIF2(aP) may also
positively regulate autophagy [112]. Amino acids do, however,
modulate the activity of eIF2B in mammalian cells. Insulin can activate
eIF2B by inducing dephosphorylation of an inhibitory site in eIF2B
(see text), via a pathway involving PKB and the inactivation of GSK3.
Ó FEBS 2002 Control of translation factors by nutrients (Eur. J. Biochem. 269) 5345
acid alcohols led to decreased S6K1 activity. In contrast,
neither we ([19]; A. Beugnet & C.G. Proud, unpublished
data)
8
nor others [34] have observed effects of amino acid
alcohols on targets of mTOR signalling. Inhibition of
hepatic protein synthesis by low amino acid levels seems
independent of uncharged tRNA [103].
How then are amino acids sensed? Amino acids also
regulate (repress) autophagy, e.g. in the liver, and this
prompted a number of studies into the mechanisms involved
in this effect. Byproducts of leucine metabolism seem
unlikely to be involved here (see discussion of [31])
suggesting a role for leucine itself. Mortimore et al.
[104,105] were able to show that a non-cell-permeant leucine
ÔanalogueÕ could still inhibit autophagy, implying that the
effect was mediated by extracellular leucine; they went on to
use photoaffinity labelling to show that this reagent could
label a membrane-associated protein, suggesting the exist-
ence of a plasma membrane leucine ÔsensorÕ [105]. This idea
is attractive in view of the discovery of a plasma membrane
amino acid sensor (Ssy1p) in yeast [106], but the molecular

important role in regulating insulin synthesis in this tissue
(see [113]).
FUTURE DIRECTIONS
The last four years or so have seen several important
advances in our understanding of the control of translation
factors by nutrients. However, as is so often the case, the
recent data raise even more questions. Particularly import-
ant issues concerning the role of mTOR signalling in the
control of translation factors include: the nature of the
machinery by which amino acids are sensed in mammalian
cells, and how this information is relayed to mTOR and the
links between mTOR and the control of the S6Ks and
4E-BPs. For example, it is important to identify the protein
kinases that act on 4E-BP1 and the S6Ks. The complex
hierarchy of phosphorylation of 4E-BP1, in particular,
suggests that multiple kinases are involved, some of which
may be basally active (due to an input from mTOR) while
others may be turned on by insulin. The role of protein
phosphatases in the control of 4E-BPs and S6Ks also needs
to be explored. In a wider context, it is crucial to identify the
other regulatory mechanisms, independent of mTOR, that
function to activate protein synthesis in muscle, for example,
in response to feeding.
ACKNOWLEDGEMENTS
Work in the author’s laboratory is supported by the Biotechnology and
Biological Sciences Research Council, the British Heart Foundation,
The European Union, The Medical Research Council and the
Wellcome Trust.
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