Downloaded from http://cshperspectives.cshlp.org/ on September 25, 2021 - Published by Cold Spring Harbor Laboratory Press Phosphorylation and Signal Transduction Pathways in Translational Control Christopher G. Proud Nutrition & Metabolism, South Australian Health & Medical Research Institute, North Terrace, Adelaide SA5000, Australia; and School of Biological Sciences, University of Adelaide, Adelaide SA5000, Australia Correspondence: [email protected] Protein synthesis, including the translation of specific messenger RNAs (mRNAs), is regulated by extracellular stimuli such as hormones and by the levels of certain nutrients within cells. This control involves several well-understood signaling pathways and protein kinases, which regulate the phosphorylation of proteins that control the translational machinery. These path- ways include the mechanistic target of rapamycin complex 1 (mTORC1), its downstream effectors, and the mitogen-activated protein (MAP) kinase (extracellular ligand-regulated kinase [ERK]) signaling pathway. This review describes the regulatory mechanisms that control translation initiation and elongation factors, in particular the effects of phosphoryla- tion on their interactions or activities. It also discusses current knowledge concerning the impact of these control systems on the translation of specific mRNAs or subsets of mRNAs, both in physiological processes and in diseases such as cancer. he control of protein synthesis plays key roles Accordingly, sophisticated control mecha- Tin cell growth and proliferation and in nisms exist to allow extracellular stimuli (e.g., many other processes, via shaping the cellular hormones, growth factors), intracellular metab- proteome. The importance of translational con- olites (essential amino acids, nucleotides) and trol is underscored, for example, by the lack of cues, such as energy status, to regulate protein concordance between the transcriptome and the synthesis. These mechanisms include control of proteome—which, among other factors, reflects the translation of specific mRNAs, via signaling the differing efficiencies with which different pathways that impinge on components of the messenger RNAs (mRNAs) are translated into initiation or elongation machinery, which is polypeptides (Schwanhausser et al. 2011; Li mainly achieved through protein kinases that et al. 2014)—and the role of dysregulation of phosphorylate and thereby regulate translation translation in human diseases, including cancer initiation or elongation factors, and certain (Robichaud et al. 2018a), virus infection (Stern- other proteins. Ginossar et al. 2018), and many others (Tahma- sebi et al. 2018). Furthermore, since mRNA CAVEATS translation consumes a substantial fraction of cellularmetabolicenergyandanevenhigherpro- Studies on the role of specific signaling pathways portion of amino acids, it is crucial that cells or their target phosphoproteins rely heavily on match the rate of protein synthesis to the avail- the use of small molecule inhibitors, especially of ability of nutrients. protein kinases, and on testing proteins in which Editors: Michael B. Mathews, Nahum Sonenberg, and John W.B. Hershey Additional Perspectives on Translation Mechanisms and Control available at www.cshperspectives.org Copyright © 2018 Cold Spring Harbor Laboratory Press; all rights reserved Advanced Online Article. Cite this article as Cold Spring Harb Perspect Biol doi: 10.1101/cshperspect.a033050 1 Downloaded from http://cshperspectives.cshlp.org/ on September 25, 2021 - Published by Cold Spring Harbor Laboratory Press C.G. Proud a given phosphorylation site has been mutated to Conversely, phosphomimetic mutants of Ser one that cannot be phosphorylated ora negative- or Thr to Glu or Asp may sometimes mimic ly charged one, which may mimic a phosphory- the addition of a phosphate, but since the latedresidue. Thereareimportant caveatstoboth geometry (tetrahedral vs. planar) and charge approaches. Compounds that inhibit protein (double vs. single) of phospho-Ser/Thr and kinases are often not specific for a single type Glu/Asp differ, phosphomimicry is by no means of enzyme because the ATP-binding sites of guaranteed. many kinases are similar. For example, the com- pounds often used as inhibitors of the mitogen- mTORC1 SIGNALING activated protein (MAP) kinase-interacting ki- nases (MNKs, which phosphorylate eukaryotic One major pathway that regulates the transla- initiation factor [eIF]4E), CGP57380 (Tschopp tional machinery in several ways is the mTOR et al. 2000), and cercosporamide (Konicek et pathway. mTOR forms two types of multi- al. 2011), affect other protein kinases (Bain et subunit complexes, mTORC1 and mTORC2. al. 2007; Beggs et al. 2015), several of them mTORC1 phosphorylates proteins that directly more potently than their effects on MNK1 and regulate the translational apparatus as well as MNK2. It is, therefore, important to test chemi- protein kinases that phosphorylate and regulate cally distinct entities that affect the kinase of in- translation factors (Saxton and Sabatini 2017). terest and, wherever possible, to complement In addition to mTOR, mTORC1 contains such work with genetic studies using cells in Raptor, mLst8, and Rheb, a GTPase (Ben-Sahra which the enzyme of interest has been knocked and Manning 2017). Raptor and Rheb are found out. Genetic studies have been made much more only in mTORC1, not in mTORC2. Raptor tractable by the advent of clustered regularly in- binds substrate proteins, conferring specificity, terspaced short palindromic repeats (CRISPR)/ whereas Rheb activates mTORC1 when it is Cas9 genome editing, which can be applied to bound to GTP. Rheb provides a key link to ex- cell lines as well as to studies in animals. tracellular stimuli through the phosphatidylino- Rapamycin is a macrolide compound that is sitide 3-kinase (PI3K) or classical MAP kinase a highly specific inhibitor of the protein kinase (extracellular ligand-regulated kinase [ERK]) complex termed mechanistic (or mammalian) signaling pathways (Fig. 1) (Saxton and Sabatini target of rapamycin complex 1 (mTORC1). Ra- 2017). Rheb is regulated by the tuberous sclero- pamycin is specific largely because it does not sis complex (TSC), which includes the proteins target the active site of mTOR, but instead binds TSC1, TSC2, and TBC1D7 (Dibble et al. 2012). (together with its partner, FKBP12, an immu- TSC2 acts as a GTPase activator protein for nophilin) to another domain, thereby partially Rheb, promoting its conversion to its inactive occluding access to the active site for some GDP-bound state. Protein kinases within or protein substrates (Yang et al. 2013). Although downstream of these pathways, Akt (also called in the short term its effects are limited to inhib- protein kinase B [PKB]), ERK, or the ERK- iting mTORC1, longer-term exposure of cells substrates termed RSKs, phosphorylate the to rapamycin also impairs mTORC2 signaling TSC complex and impair its ability to promote by interfering with assembly of this complex GTP hydrolysis on Rheb. This allows stimuli (Sarbassov et al. 2006). that switch on PI3K or ERK signaling to en- The effects of mutating a phosphorylation hance the levels of Rheb-GTP and thereby acti- site of interest must also be treated with caution. vate mTORC1. Nonphosphorylatable mutants are generally Several components of the PI3K and ERK created by mutating the relevant serine or pathways undergo gain-of-function oncogenic threonine to alanine, which, because it lacks mutations in cancers, whereas others are tumor a hydroxyl group, cannot accept a phosphate. suppressors whose function is lost in some However, alanine also differs from serine and cancers. These pathways, and hence mTORC1 threonine in being unable to form H-bonds. signaling, are constitutively activated in a high 2 Advanced Online Article. Cite this article as Cold Spring Harb Perspect Biol doi: 10.1101/cshperspect.a033050 Downloaded from http://cshperspectives.cshlp.org/ on September 25, 2021 - Published by Cold Spring Harbor Laboratory Press Signaling and Translational Control Hormone and growth Cytokine factor receptors, etc. receptors Plasma membrane PI3K/PTEN Ras/NF1 signaling signaling P P P Akt (PKB) ERK (MAPK) p38 MAPKα/β P P P TSC1/2 P MNK1 P RSKs P + Lysosome Rheb•GTP AMPK MNK2 (low ATP) Amino acids P mTORC1 S6Ks P P Amino acids eEF2K P P P P 4E-BPs eIF4B P PDCD4 P eIF4A P LARP1 eIF4E eEF2 ? ?? ? ? ?? ? ? TOP TISU mRNAs with long/ Specific General mRNAs mRNAs structured 5′UTRs mRNAs translation Initiation Elongation Figure 1. Major signaling connections to the translational machinery. This simplified schematic shows the main signaling links to the translational machinery. Solid lines depict direct links and dashed lines indirect ones (involving multiple components, for example). (P) indicates phosphorylation events, green indicates activation, red indicates an inhibitory phosphorylation, and gray denotes that the functional consequence is unclear. Not all the components or phosphorylation sites mentioned in the text are shown. Question marks denote some of the open questions described in the text. percentage of human cancers, estimated at 70% surface, where AMPK can be activated (Lin or higher (Saxton and Sabatini 2017). and Hardie 2017). This happens, for example, mTORC1 signaling is also activated by ami- when cellular ATP levels fall and levels of ADP, no acids, at the surface of the lysosome (Fig. 1), or especially AMP, rise. Recent studies have by sophisticated sensing and signaling mecha- identified an