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Supplement: 2nd Amino Acid Workshop

Amino Acids as Regulators of Gene Expression at the Level of mRNA Translation^1,2

Department of Cellular and Molecular Physiology, Pennsylvania State University College of Medicine, Hershey, PA 17033

³ To whom correspondence should be addressed. E-mail: jjefferson{at}psu.edu.

	ABSTRACT

TOP ABSTRACT LITERATURE CITED

 
Amino acids act through a number of signaling pathways and mechanisms to mediate control of gene expression at the level of mRNA translation. This report reviews recent findings that illustrate the manner through which amino acids act to regulate the initiation phase of mRNA translation. The report focuses on signaling pathways that involve the eukaryotic initiation factor-2 (eIF2) protein kinase, general control non-derepressing kinase-2 and the mammalian target of rapamycin (mTOR) protein kinase. It also describes the mechanisms through which amino acid–induced modulation of eIF2 phosphorylation and mTOR-mediated signaling cause derepression of translation of specific mRNAs and result in an overall change in the pattern of gene expression. Finally, it provides examples of mRNAs whose translation is modulated through these mechanisms.

KEY WORDS: • mRNA leucine • upstream open reading frame • internal ribosome entry site • eukaryotic initiation factor

A growing number of examples illustrate that many and perhaps all genes are regulated at multiple steps that include transcription, posttranscriptional processing, nuclear export, stability and translation of mature mRNA molecules. Translation itself is regulated by a diverse array of mechanisms that act not only at the initiation step but also during elongation and termination. This review focuses on the initiation step and some of the regulatory mechanisms involved in the discrimination as to which mRNA are selected for translation and to what extent they are translated into protein. Moreover, it focuses specifically on amino acids as nutrient regulators of initiation and the signaling pathways through which amino acids exert their effects on translation.

Phosphorylation of eukaryotic initiation factor-2 as a regulatory mechanism for the translational control of gene expression

Phosphorylation of the -subunit of eukaryotic initiation factor-2 (eIF2, a trimeric complex composed of -, ß- and -subunits)⁴ on Ser-51 has been recognized for a number of years as a mechanism for suppressing global protein synthesis [reviewed in (1)]. The phosphorylation event is mediated by at least four different protein kinases, all of which are activated in response to some specific type of cellular stress. For example, the protein kinase known as PERK [double-stranded RNA-activated protein kinase (PKR)-like endoplasmic reticulum-associated kinase] is activated in response to any number of conditions, and all of them result in malfunctioning of the process of protein folding in the endoplasmic reticulum (2,3). PKR and heme-regulated inhibitor are activated by double-stranded RNA during viral infection and hemin deprivation, respectively (4,5). Finally, general control non-derepressing kinase-2 (GCN2), which is an eIF2 kinase that is conserved from yeast to mammals, is activated in response to amino acid starvation, purine limitation or DNA damage (6–10). Acting alone or in combination, the eIF2 kinases mediate hyperphosphorylation of Ser-51 in response to a variety of stress conditions including suboptimal levels of amino acids, glucose or serum; exposure to heat, heavy metals or arsenite; formation of free oxygen radicals or hypoxic or hyperosmotic conditions [c.f. (10)]. Hyperphosphorylation of eIF2 leads to suppression of global protein synthesis through a mechanism whereby binding of the initiator met-tRNA_i to the 40S ribosomal subunit is inhibited [reviewed in (1)]. Initiator met-tRNA_i joins the 40S ribosomal subunit as a ternary complex with eIF2 and GTP. After each cycle of initiation, the eIF2 is released as a binary complex with GDP, and for eIF2 to function in another round of initiation, the GDP must be exchanged for GTP. This guanine nucleotide exchange reaction is catalyzed by another initiation factor, eIF2B. When eIF2 becomes phosphorylated, it forms an unproductive complex with eIF2B, the binding of initiator met-tRNA_i to the 40S ribosomal subunit becomes restricted and global protein synthesis is suppressed.

Although phosphorylation of eIF2 leads to a suppression of global protein synthesis, accumulating evidence demonstrates that it also results in the derepression of translation of a number of specific mRNA. The first evidence in support of this type of role for phosphorylated eIF2 was provided by studies in the yeast Saccharomyces cerevisiae in which it was shown that GCN2 mediates activation of the general amino acid control pathway when one or more amino acids becomes limiting for growth [reviewed in (11)]. It does so by mediating the derepression of translation of the mRNA for GCN4, which is a transcriptional activator of genes that are involved in amino acid biosynthesis and related metabolic pathways. GCN4 mRNA contains a unique cluster of upstream open reading frames (uORF) that mediate translational derepression of the GCN4 coding sequence only when a certain threshold level of eIF2 phosphorylation is achieved as a result of activation of GCN2. Limitation of amino acids is thought to activate GCN2 through a mechanism that involves accumulation of deacylated tRNA.

The first evidence that the mammalian ortholog of GCN2 is also activated by deacylated tRNA is provided by a recent report (12) in which liver from wild-type (GCN2^+/+) mice or mice with a chromosomal disruption of the GCN2 gene (GCN2^-/-) were perfused in situ with medium that contained or lacked the histidinyl–tRNA synthetase inhibitor, histidinol. In liver from GCN2^+/+ mice, histidinol treatment resulted in enhanced phosphorylation of eIF2 as well as inhibition of eIF2B activity. In contrast, in liver from GCN2^-/- mice, histidinol had no effect on either eIF2 phosphorylation or eIF2B activity. It is noteworthy that in the absence of histidinol, eIF2 phosphorylation was lower and eIF2B activity was higher in liver from GCN2^-/- compared with GCN2^+/+ mice, which suggests that under these conditions, GCN2 may be the predominant eIF2 kinase activity in mouse liver. The same report (12) showed that phosphorylation of eIF2 in response to leucine deprivation of mouse embryonic stem cells was also dependent on GCN2.

Besides the yeast GCN4, there is a growing list of examples of genes that are regulated by uORF. Some well-characterized transcription factor examples include activating transcription factor 4 (7), CCAAT/enhancer binding proteins- and -ß (13); the oncoprotein mouse double-minute 2, which is a part of a negative-feedback loop that regulates the activity of the tumor suppressor p53 (14) and the human epidermal growth factor receptor-2 (15); S-adenosylmethionine decarboxylase, which is a key enzyme in the biosynthetic pathway for the polyamines spermidine and spermine (16,17); and ß₂-adrenergic (18), retinoic acid (19), glucocorticoid (20) and estrogen (21) receptors. Thus, uORF appear frequently in genes with critical biological functions.

An alternative mode of translational control of gene expression mediated by eIF2 phosphorylation involves recruitment of the translation initiation complex by an internal ribosome entry site (IRES). Translation by internal ribosome entry was first identified in picornaviruses (22,23), but a number of cellular mRNA have subsequently been found to contain an IRES including vascular endothelial growth factor (24), hypoxia-inducible factor-1 (25), protein kinase C- (26), basic fibroblast growth factor (27), c-myc (28), X-linked inhibitor of apoptosis (29), p97 [also known as death-associated protein or N-acetyltransferase-1 (30)] and ornithine decarboxylase (31). Although the advantages for viruses to contain an IRES are quite clear (i.e., 5' cap-independent initiation), the advantages for a cellular mRNA IRES were not immediately understood. However, it is now becoming recognized that the 5' untranslated regions of many cellular mRNA that serve in critical biological functions are long and structured and contain upstream AUG codons, so that scanning ribosomes are unlikely to efficiently initiate translation (32). Efficient initiation of the translation of these mRNA in many examples including some of those listed above has been shown to be mediated by an IRES. Recent evidence (33,34) suggests that phosphorylation of eIF2 is required for activation of IRES-mediated translation initiation. Whereas production of poorly translated mRNA seems inefficient, evolution has clearly tolerated and apparently exploited uORF and IRES for regulatory purposes. As future work unfolds, it will certainly be exciting to identify the genes regulated by amino acid availability through these novel regulatory elements.

Phosphorylation of eIF4E binding protein-1, ribosomal protein S6 and eIF4G as regulatory mechanisms for translational control of gene expression

Three proteins involved in the joining of mRNA with the 40S ribosomal subunit, i.e., eIF4E-binding protein-1 (4E-BP1), ribosomal protein S6 (rpS6) and eIF4G, are downstream targets of a signaling pathway that includes the protein kinase referred to as the target of rapamycin (TOR) [reviewed in (35)]. TOR was first identified in S. cerevisiae as two genes (TOR-1 and -2) that permit growth in the presence of the immunosuppressant rapamycin [reviewed in (35)]. Subsequent studies revealed that TOR orthologs are present in flies, worms and mammals. The TOR proteins are members of a family of proteins that have in common a C-terminal protein kinase domain with homology to the catalytic domain of phosphatidylinositol 3-kinase (PI3K). Members of the PI3K-related family of kinases also contain a FAT (FRAP, ATM, TRRAP) domain that is speculated to serve as a site that might be involved in protein-protein interaction. In addition to the catalytic and FAT domains, TOR contains multiple HEAT (Huntingtin eIF3, A-subunit of protein phosphatase 2A, TOR) motifs which, like the FAT domain, are thought to be involved in protein-protein interaction. Finally, the TOR protein also has an FK506 binding protein (FKB)-rapamycin binding domain that binds the FKB protein-12·rapamycin complex and mediates inhibition of TOR activity by rapamcyin.

In yeasts and mammals, one of the major functions of TOR is to coordinate nutrient availability with cell growth and proliferation [reviewed in (36,37)]. In mammalian cells, mammalian TOR (mTOR) is involved in regulating the binding of mRNA to the 40S ribosomal subunit [reviewed in (35)]. The mRNA binding step in translation initiation is mediated by a complex of translation initiation factors that is referred to as eIF4F and is comprised of eIF4A (an RNA helicase), eIF4E (the protein that binds to the m⁷GTP cap structure at the 5' end of mammalian cytoplasmic mRNA) and eIF4G, a scaffolding protein that in addition to binding to eIF4A and eIF4E binds to the poly(A) binding protein (PABP) and eIF3 [reviewed in (38)]. The binding of eIF4G to eIF3 is of particular importance because it is through this interaction that the eIF4F·mRNA complex binds to the 40S ribosomal subunit. Assembly of the eIF4F complex is regulated in part through the association of eIF4E with the so-called eIF4E binding proteins (4E-BP), of which 4E-BP1 is the prototype. The binding site on eIF4E for 4E-BP1 overlaps with that for eIF4G, such that either 4E-BP1 or eIF4G can bind individually to eIF4E, but both cannot bind at the same time (39). Thus, binding of eIF4E to 4E-BP1 prevents mRNA from binding to the 40S ribosomal subunit. Formation of the 4E-BP1·eIF4E complex only occurs when 4E-BP1 is hypophosphorylated; hyperphosphorylated forms of the protein do not bind to eIF4E. Of particular relevance to this article is the finding that the hyperphosphorylation of 4E-BP1 that occurs in response to provision of amino acids to deprived cells in culture, and in particular to provision of leucine to amino acid–deprived cells, is blocked by rapamycin (40,41). A similar effect is observed in fasted rats, where either provision of dietary protein (42) or oral administration of leucine (43) promotes 4E-BP1 hyperphosphorylation and assembly of the eIF4G·eIF4E complex in skeletal muscle. A similar effect of amino acids on 4E-BP1 phosphorylation is observed in skeletal muscle of pigs (44) and humans (45). However, administration of rapamycin before feeding or administering oral leucine completely prevents the increase in 4E-BP1 phosphorylation (46,47) and thereby implicates mTOR in the response.

Another control point in the mRNA binding step in translation initiation that is regulated by mTOR involves changes in phosphorylation of rpS6. Phosphorylation of rpS6 occurs on multiple sites and is mediated by a 70-kDa protein kinase termed S6K1 [reviewed in (48)]. The activity of S6K1 is itself regulated by multisite phosphorylation, whereby phosphorylation of sites in the C-terminus of the protein permits phosphorylation of other internal sites such as Ser-229 and Thr-389. Phosphorylation of the internal sites is critical for optimal activation of the kinase. Thr-389 is phosphorylated in vitro by mTOR (49) and treatment of cells in culture [c.f. (40)] and animals in vivo (46) with rapamycin prevents amino acid–induced phosphorylation of the residue.

Early studies suggested that phosphorylation of rpS6 might enhance global protein synthesis. However, recent studies suggest that rather than controlling translation of mRNA in a global manner, phosphorylation of rpS6 enhances the translation of a subset of mRNAs that contain a common structural feature: an uninterrupted stretch of 7–15 pyrimidine residues that are adjacent to the 5' cap structure [terminal oligopyrimidine tract (TOP) mRNA; reviewed in (50)]. Proteins encoded by such mRNAs include the ribosomal proteins, eIF4G, PABP and eukaryotic elongation factor-2; i.e., proteins that are involved in mRNA translation. Related studies have shown that ribosomal DNA transcription is also inhibited by rapamycin (51). Thus, mTOR controls the synthesis of both ribosomal proteins and ribosomal RNA; in other words, mTOR controls ribosome biogenesis. It should be noted that a recent report (52) questions the assumptions that activation of S6K1 and phosphorylation of rpS6 are required for upregulated translation of TOP mRNA. In the study reported, amino acid stimulation of TOP mRNA translation was dependent upon intact signaling through PI3K but was not dependent upon either S6K1 or phosphorylation of rpS6. Thus, there may exist multiple signaling pathways, one or more of which may be independent of mTOR, that can regulate TOP mRNA translation.

Evidence suggesting that amino acids may regulate a step in translation initiation that does not depend on rapamycin-sensitive signaling through mTOR is provided by a study wherein fasted rats were treated with rapamycin before oral administration of leucine (46). In control animals, leucine stimulated protein synthesis, phosphorylation of 4E-BP1 and S6K1 and binding of eIF4G to eIF4E. As noted above, rapamycin treatment prevented the leucine-induced phosphorylation of 4E-BP1 and S6K1. However, rapamycin attenuated but did not prevent the stimulation of protein synthesis or assembly of the eIF4F complex that was caused by leucine administration. A similar result was reported for skeletal muscle of perfused rat hindlimb preparations whereby increasing the concentrations of amino acids in the perfusate from those observed in fasted rats to 10 times those amounts stimulated protein synthesis and the binding of eIF4G to eIF4E but had no effects on 4E-BP1 or S6K1 phosphorylation (53).

Recent evidence suggests that insulin is required for amino acid–induced signaling through mTOR. For example, in diabetic rats, oral leucine administration enhances protein synthesis in skeletal muscle, although the absolute rate is only 50% of the value observed in control animals that are administered leucine (54). Moreover, unlike in control animals, in diabetic rats, leucine is unable to stimulate eIF4G binding to eIF4E or phosphorylation of 4E-BP1 or S6K1. In muscle from diabetic rats, phosphorylation of protein kinase B (PKB) is almost undetectable. Infusion of insulin at a rate sufficient to restore the plasma insulin concentration to a value equivalent to that observed in plasma from a fasted control rat restores PKB phosphorylation to levels observed in a fasted control animal. Although leucine does not stimulate PKB phosphorylation (even in a control animal), oral leucine administration to diabetic rats infused with insulin enhances the binding of eIF4G to eIF4E as well as phosphorylation of 4E-BP1 and S6K1. The idea that both insulin and amino acids are required for activation of the mRNA binding step is also supported by results from a study in which diazoxide was used to acutely attenuate insulin secretion (55). In that study, protein synthesis, 4E-BP1 and S6K1 phosphorylation and eIF4G binding to eIF4E were all enhanced by feeding fasted rats a protein-containing but not protein-free meal. However, in fasted rats treated with diazoxide, the protein-containing meal had no effect on any of these variables. The result of these and similar studies suggest that by maintaining a minimal level of activation of the PI3K-PKB signaling pathway, insulin has a permissive effect on signaling by amino acids to targets downstream of mTOR.

Mechanisms for regulation of mTOR signaling: TOR-interacting proteins

One mechanism through which the activity of mTOR can be modulated is through interactions with other proteins. Two proteins recently shown to repress mTOR signaling are tuberous sclerosis complexes (TSC)-1 and -2 (56). The genes for TSC1 and TSC2 encode proteins (hamartin and tuberin, respectively) that were originally identified as tumor suppressor genes in humans, and mutations in either gene are associated with the growth of disorganized benign tumors in a variety of organs (57,58). Such tumors are characterized by the presence of abnormally large cells.

TSC1 and TSC2 are also present in Drosophila melanogaster. Mutations in the D. melanogaster TSC2 gene result in the Gigas phenotype; like tuberous sclerosis in humans, this phenotype is characterized by an unusually large cell size [reviewed in (59)]. Genetic analysis in D. melanogaster place the TSC1·TSC2 complex downstream of PKB and upstream of S6K1 in the insulin signal transduction pathway (60,61). Indeed, two recent studies (62,63) report that PKB directly phosphorylates TSC2 on at least two residues, Ser-924 and Thr-1518, in D. melanogaster and mammalian cells. A more recent genetic analysis places TSC1 and TSC2 upstream of TOR (56). In fact, both proteins bind to mTOR when coexpressed in D. melanogaster S2 cells and inhibit mTOR signaling to S6K1 and 4E-BP1 (56). The mechanism by which phosphorylation of TSC2 results in inhibition of TOR may involve dissociation of the TSC1·TSC2 complex (62). Thus, phosphorylation of TSC2 by PKB results in dissociation of the complex and rapid degradation of the protein (63). Moreover, an exogenously expressed variant of TSC2 wherein the two PKB phosphorylation sites were changed to acidic residues to mimic phosphorylation does not bind to TSC1 and is unstable (62). However, it should be noted that in a different study (64), phosphorylation of TSC2 by PKB did not affect its ability to bind to TSC1, which suggests that additional studies in this area are warranted.

Further evidence implicating TSC1 and TSC2 in modulating signaling through mTOR is provided by studies in which the expression of the proteins is exogenously enhanced or suppressed. Suppressed expression of either TSC1 or TSC2 results in constitutive hyperphosphorylation and activation of S6K1 that is sensitive to rapamycin (62,65,66), which supports the idea that TSC1 and TSC2 act upstream of mTOR. Furthermore, RNA_i-mediated suppression of TSC2 expression results in enhanced phosphorylation of mTOR on Ser-2448 (62). In contrast, when both proteins are simultaneously overexpressed in human embryonic kidney (HEK)-293 cells, phosphorylation of S6K1 and 4E-BP1 is low relative to control cells (62,67). However, expression of either TSC1 or TSC2 alone has little if any effect on phosphorylation of S6K1 or 4E-BP1, which indicates that the two proteins function as a complex. The phosphorylation sites on S6K1 and 4E-BP1 that are affected by TSC1 and TSC2 overexpression are those that have been demonstrated to be phosphorylated by mTOR in vitro; e.g., Thr-389 on S6K1 and Thr-36, Thr-45, Ser-65 and Thr-70 on 4E-BP1. Mutation of the PKB phosphorylation sites on TSC2 to Ala enhances the ability of the protein to suppress mTOR signaling to S6K1 and 4E-BP1, whereas mutation to acidic residues to mimic phosphorylation of the protein represses the effectiveness of the protein in inhibiting mTOR signaling (62,64). This demonstrates the important role played by TSC2 phosphorylation in regulating signaling through mTOR. Finally, rapamycin-resistant variants of S6K1 are also resistant to overexpression of TSC1 and TSC2 (62). Overall, the results are consistent with a model whereby the TSC1·TSC2 complex binds to mTOR and prevents PKB-mediated phosphorylation and activation of mTOR. However, this model may be insufficient to explain the observation that TSC1 and TSC2 repress not only PKB signaling to targets downstream of mTOR, but also suppress amino acid signaling to such targets (56,67). Amino acids do not activate PKB, although they do promote phosphorylation of Ser-2448 on mTOR. Thus, amino acids may promote dissociation of the TSC1·TC2 complex from mTOR, which would subsequently allow PKB access to Ser-2448. Such a mechanism would not require activation of PKB but would require release of the TSC1·TSC2 complex.

In addition to TSC1 and TSC2, several other recently identified proteins were shown to associate with and regulate the activity of mTOR [reviewed in (68)]. For example, the regulatory associated protein of mTOR (raptor) regulates both the activity of mTOR and its sensitivity to rapamycin (69,70). Although the mechanism(s) through which raptor modulates mTOR activity is still incompletely defined, it is clear that it binds to mTOR and that such an association is required for mTOR to efficiently phosphorylate 4E-BP1 and S6K1. Thus, suppression of raptor expression in either HeLa (69) or HEK-293T (70) cells results in decreased phosphorylation of 4E-BP1 and S6K1, respectively. Such studies also revealed that raptor has a positive role in nutrient signaling because suppression of raptor expression represses leucine-stimulated activation of S6K1 (70).

In addition to regulating the activity of mTOR toward downstream targets, raptor also may be a determinant in controlling the sensitivity of mTOR to rapamycin. In S. cerevisiae, five TOR-interacting proteins were recently identified: KOG1, LST8, AVO1, AVO2 and AVO3 (71). In yeasts, the TOR2 isoform can bind to either KOG1 or the AVO proteins, whereas LST8 is present in both the TOR·KOG1 and TOR·AVO1·AVO2·AVO3 complexes. In contrast, the TOR1 isoform only binds to KOG1 and LST8. Interestingly, when bound to KOG1, the activities of TOR1 and TOR2 are sensitive to inhibition by rapamycin, but when TOR2 is bound to the AVO proteins, it is rapamycin insensitive. Thus, at least two independent TOR–signaling pathways exist in yeast: one that is repressed by rapamycin and one that is not repressed. Whether the rapamycin-independent signaling pathway is present in mammals is unknown. However, mammalian orthologs of the KOG1, LST8 and AVO1 proteins have been identified: raptor, mLST8 and SAP kinase interacting protein (SIN1), respectively, and both raptor and mLST8 coimmunoprecipitate with mTOR (71). Whether SIN1 interacts with mTOR needs to be examined in future studies.

Although the precise mechanisms through which the TOR-interacting proteins regulate the activity of the kinase have not been determined, the available evidence supports a model whereby TSC1 and TSC2 bind to mTOR and thereby suppress phosphorylation of downstream targets such as 4E-BP1 and S6K1. In this model, amino acids and insulin promote release of the TSC1·TSC2 complex from mTOR and permit PKB to phosphorylate the kinase on Ser-2448. In addition, release of the TSC1·TSC2 complex allows mTOR to bind to LST1 and raptor, which is an event that enhances the ability of the kinase to phosphorylate 4E-BP1 and S6K1. Release of the TSC1·TSC2 complex might also allow mTOR to bind to SIN1 and result in activation of an undetermined rapamycin-insensitive signaling pathway(s).

In summary, as is evident from the studies described, amino acids regulate gene expression through multiple mechanisms that include modulation of eIF2B activity, change in eIF4F assembly,and alteration of phosphorylation of rpS6 (see Fig. 1). In each case, the translation of mRNAs that encode individual proteins is enhanced or repressed based upon specific structural features present in the 5' untranslated region of the mRNA. For example, in the case of rpS6 phosphorylation, mRNA that contain a TOP sequence adjacent to the 5' cap structure are preferentially translated. For those targets downstream of mTOR such as rpS6, amino acid–induced translation of specific mRNA may also require that a basal amount of insulin (i.e., the concentration observed in plasma of fasted animals) be available so that protein kinases upstream of mTOR are at least minimally activated. The recent identification of proteins that interact with and appear to modulate the activity and substrate specificity of mTOR is an exciting development that in the future may provide the answer to the as-yet unanswered question of how amino acids regulate the function of targets downstream of mTOR.

View larger version (27K): [in this window] [in a new window]   FIGURE 1  Regulation of gene expression through modulation of translation of mRNA that encode specific proteins by amino acids and insulin. The diagram highlights the mechanisms through which amino acids regulate gene expression through modulation of mRNA translation. Details of the individual steps are discussed in the text. eIF, eukaryotic initiation factor; eIF2(P), eIF2 phosphorylated on residue 51 of its -subunit; BP1, eIF4E binding protein-1; GCN2, general control non-derepressing kinase-2 amino acid–regulated eIF2 kinase; GSK3, glycogen synthase kinase-3; PKB, protein kinase B; mTOR, mammalian target of rapamycin protein kinase; S6K1, 70-kDa ribosomal protein S6 protein kinase; uORF, upstream open reading frame; UTR, untranslated region.

	FOOTNOTES

² The studies described in this article that were performed in the laboratories of the authors were supported by research grants DK-13499 and DK-15658 from the National Institutes of Health.

⁴ Abbreviations used: 4E-BP1, eIF4E binding protein-1; eIF, eukaryotic initiation factor; GCN, general control non-derepressing; FAT, FRAP-ATM-TRRAP; IRES, internal ribosome entry site; mTOR, mammalian target of rapamycin; PI3K, phosphatidylinositol 3-kinase; PKB, protein kinase B; raptor, regulatory associated protein of mTOR; rpS6, ribosomal protein S6; S6K1, 70-kDa ribosomal protein S6 kinase; SIN1, stress-associated protein kinase interacting protein; TOP, terminal oligopyrimidine tract; TSC, tuberous sclerosis complex; uORF, upstream open reading frame.

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