The amino acid sensitive TOR pathway from yeast to mammals
Stephen G. Dann*, George Thomas
University of Cincinnati Genome Research Institute, 2180 East Galbraith Road, Cincinnati, OH 45237, USA
Received 16 April 2006; accepted 24 April 2006
Available online 2 May 2006
Edited by Horst Feldmann
of cell growth that integrates signals from growth factors and
nutrients. Two downstream effectors of mammalian TOR, the
translational components S6K1 and 4EBP1, are commonly used
as reporters of mTOR activity. The conical signaling cascade ini-
tiated by growth factors is mediated by PI3K, PKB, TSC1/2 and
Rheb. However, the process through which nutrients, i.e., amino
acids, activate mTOR remains largely unknown. Evidence exists
for both an intracellular and/or a membrane bound sensor for
amino acid mediated mTOR activation. Research in eukaryotic
models, has implicated amino acid transporters as nutrient sen-
sors. This review describes recent advances in nutrient signaling
that impinge on mTOR and its targets including hVps34, class
III PI3K, a transducer of nutrient availability to mTOR.
? ? 2006 Federation of European Biochemical Societies. Published
by Elsevier B.V. All rights reserved.
The target of rapamycin (TOR) is an ancient effector
Keywords: mTOR; Amino acid transport; Leucine; hVPS34
1. Introduction: Insulin stimulation and mTOR signaling
The coordinated control of cell growth to produce a geneti-
cally predetermined cell size, organ shape and body plan is lar-
gely directed by the mammalian target of rapamycin (mTOR).
A large protein of ?280 kDa, mTOR consists of a number of
Huntington, EF3, A subunit of PP2A, and TOR1 repeats
Frap, ATM, and TRAP PIKK-like domain (FAT domain); a
nal kinase domain; and two regulatory domains, termed the
negative regulatory domain (NRD Domain) and FAT domain
C-terminal (FAT/C Domain) (reviewed in ). The kinase do-
main is similar to the phosphatidylinositol 3OH-kinase (PI3K)
domain and in mammals mTOR was originally considered a
phosphotidylinositol-4 kinase . Further research, however,
showed that TOR was in fact a protein kinase, belonging to
the PI3K-related family of protein kinases, which also includes
ATM, ATR and DNA-dependent protein kinase .
Stimulation of PI3K by growth factors such as insulin re-
sults in the initiation of a number of signaling cascades that
lead to growth and proliferation, a cellular phenomenon con-
served throughout metazoans (Fig. 1). In mammals, receptor
interaction with insulin results in recruitment of insulin recep-
tor substrates (IRS) to the cell membrane . Subsequently,
recruitment and stimulation of the class I PI3K produces the
phosphatidylinositol second messenger PIP3 . PIP3 binds
to the pleckstrin homology (PH) domain of several proteins,
in particular to the Protein A, G and C (AGC) serine/threo-
nine kinase PKB/AKT, a pro-growth, pro-survival kinase .
Binding of PIP3to its PH domain recruits PKB to the cell
membrane where it is activated through phosphorylation by
PDK1 and PDK2 [7,8]. Activated PKB phosphorylates the
TSC1/2 complex resulting in their dissociation and degrada-
tion, thereby releasing the small GTPase Rheb from the inhib-
itory GAP activity of TSC2 [9–13]. Recent studies illustrate a
direct interaction between Rheb and mTOR, which stimulates
its kinase activity .
Two alleles, TOR1p and TOR2p, were originally described
in a screen of yeast mutants resistant to toxic doses of the
anti-fungal, bacterial macrolide, rapamycin . In mammals,
rapamycin blocks mTOR function by forming an inhibitory
complex with the immunophilin FKBP12, which binds to
and attenuates the ability of mTOR to phosphorylate down-
stream substrates [16,17], including S6K1 [3,18–20] as well as
the 4E binding protein, 4E-BP1 [21–23], a repressor of transla-
tion initiation factor 4E . In this rapamycin sensitive path-
way mTOR is bound to two additional proteins, raptor and
mLst8/GbL, to make Complex 1. Raptor and mLst8 are
homologues of the yeast KOG1p and Lst8p, respectively
[25–27]. mTOR interacts with downstream substrates through
raptor, which recognizes mTOR substrates through their TOR
signaling (TOS) motifs , whereas mLST8 is required to
make a competent signaling complex that can respond to
nutrient and energy inputs . Recent studies show that
mTOR also exists in a second signaling complex with mLST8
and a protein termed rictor, rather than raptor. Moreover, this
second complex is proposed to directly control PKB phosphor-
ylation and activation and, unlike Complex 1, Complex 2 is
rapamycin resistant [8,29].
Activation of mTOR through both growth factor and nutri-
thesis. Phosphorylation of 4E-BP1 by mTOR induces its
dissociation from initiation factor eIF4E . Relieved of 4E-
BP1, eIF4E is free to interact with the eIF-4G subunit of the
eIF-4F complex . Once these associations are complete,
the secondary structure of mRNA can be melted allowing the
40S ribosomal subunit to scan the mRNA until it encounters
the first AUG initiation codon and promotes translation. In
Abbreviations: HEAT domains, Huntington, EF3, A subunit of PP2A,
and TOR1 Domains; FAT, Frap, ATM, and TRAP PIKK like do-
mains; FAT/C, FAT domain C-terminal; FRB, FKBP12-Rapamycin
Binding domain; NRD, Negative Regulatory Domain
*Corresponding author. Fax: +1 513 558 5061.
E-mail address: Stephen.Dann@uc.edu (S.G. Dann).
0014-5793/$32.00 ? 2006 Federation of European Biochemical Societies. Published by Elsevier B.V. All rights reserved.
FEBS Letters 580 (2006) 2821–2829
somal protein S6, whose phosphorylation was earlier shown to
be implicated in increased ratesof protein synthesis . The re-
cent identification of four additional substrates is also consis-
tent with a role in cell growth, including translation initiation
factor 4B, which facilitates the unwinding of the 50ends of
mRNA , eEF2 kinase, which mediates the phosphorylation
of translation elongation factor 2, involved in controlling ribo-
somal transit rates , BAD, a pro-apoptotic protein  and
most recently SKAR a nuclear protein proposed to couple tran-
scription with pre-mRNA splicing and mRNA export . The
role of these targets in cell growth is consistent with the effect of
the S6Ks on growth, as established in deletion studies in both
Drosophila  and in the mouse [36,37].
The ability of nutrients to modulate mTOR signaling down-
stream to its substrates is relatively well documented, however
little is known with regard to the identity of the upstream effec-
tors of this branch of the signaling pathway. Evidence exists
for both an intracellular and/or a membrane bound extracellu-
lar sensor for amino acid stimulation of the mTOR pathway.
Research in yeast, Drosophila, and mammalian settings have
implicated amino acid transporters as sensors of nutrient avail-
ability. Investigations in Drosophila and yeast have described
amino acid transporters as genetically epistatic to TOR activity
while, in a mammalian setting, mTOR activity is linked to ami-
no acid transporter transcription, translation and turnover.
Considering the well conserved nature of the TOR pathway
in eukaryotes, it seems likely that membrane nutrient trans-
porters are able to promote traditional signaling cascades be-
yond their well known function of regulating nutrient
concentration within the cell. The importance of the nutrient
branch of the mTOR signaling pathway is underscored by re-
cent studies showing that nutrient overload, while driving cell
growth, also acts to suppress insulin signaling, leading to
hyperglycemia and insulin resistance [36,38]. Moreover, obes-
ity and type 2 diabetes have recently been implicated as risk
factors in a number of cancers [39,40], consistent with the clin-
ical importance of rapamycin, and its derivatives in the treat-
ment of solid tumors [1,41]. This review will describe recent
advances in a variety of eukaryotic models of nutrient signal-
ing that impinge on the mTOR kinase and its targets including
the recent identification of hVps34, class III PI3K, as a trans-
ducer of nutrient availability to mTOR [42,43].
2. Yeast studies
Many advances in the field of cell growth control in the
mammalian setting were initially described in yeast. The
TOR1 and TOR2 alleles as well as the distinct TOR complexes
are examples of the advantages that a simple eukaryotic system
confers in describing novel pathways and potential interac-
tions. In yeast, Torp and Lst8p have been implicated in the
nutrient responsive, mitochondria-to-nucleus, retrograde sig-
naling (Fig. 2). The most detailed reports on this pathway in-
volve the de-repression of Rtg1p and Rtg3p target genes, many
of which control the supply of intermediates necessary for the
tricarboxylic acid cycle (reviewed in ). In brief, restriction
of a suitable glutamate source results in Rtg1p:Rtg3p tran-
scription complex being released from a repressed state leading
to the subsequent upregulation of target gene families, includ-
ing CIT1, CIT2, ACO1, IDH1 and IDH2 . Glutamate or
glutamine is sensed in yeast by the membrane bound SPS sen-
sor which negatively regulates Rtg2p. Rtg2p is also a sensor of
Fig. 1. Insulin and amino acid signals converge on mTOR through independent PI3K signals. Insulin binds to its receptor resulting in activation of
the class I PI3K which recruits PKB to the cell membrane. Upon activation by PDK1 and PDK2 PKB inhibits TSC1/2 which releases Rheb from its
intrinsic GTPase activity. Rheb-GTP bound to mTOR activates mTOR kinase activity resulting in phosphorylation of its downstream targets S6K1
and 4EBP1. Meanwhile amino acid availability positively regulates the activity of the class III PI3K hVPS34. hVPS34 produces the second messenger
PI(3)P which activates mTOR through unknown mechanisms.
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