The Epitranscriptome in Translation Regulation
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Downloaded from http://cshperspectives.cshlp.org/ on October 1, 2021 - Published by Cold Spring Harbor Laboratory Press The Epitranscriptome in Translation Regulation Eyal Peer, Sharon Moshitch-Moshkovitz, Gideon Rechavi, and Dan Dominissini Sackler School of Medicine, Tel Aviv University, Tel Aviv 6997801, Israel; Cancer Research Center and Wohl Centre for Translational Medicine, Chaim Sheba Medical Center, Tel-Hashomer 5262160, Israel Correspondence: [email protected] The cellular proteome reflects the total outcome of many regulatory mechanisms that affect the metabolism of messenger RNA (mRNA) along its pathway from synthesis to degradation. Accumulating evidence in recent years has uncovered the roles of a growing number of mRNA modifications in every step along this pathway, shaping translational output. mRNA modifications affect the translation machinery directly, by influencing translation initiation, elongation and termination, or by altering mRNA levels and subcellular localization. Features of modification-related translational control are described, charting a new and complex layer of translational regulation. ellular homeostasis requires regulation of introduced the novel notion that internal chem- Cprotein levels, as impaired regulation can ical modifications of mRNA and long noncod- result in cellular death or dysregulated prolifer- ing RNA (lncRNA) are potentially dynamic and ation (Hershey et al. 2012). Protein levels are a sometimes reversible events, which constitute consequence of three fundamental factors: the essential regulatory elements in basic RNA-pro- abundance of messenger RNA (mRNA), the cessing steps such as splicing, transport, trans- efficiency of translation, and protein stability. lation, and decay. The epitranscriptome, as this First isolated from ribosomal RNA (rRNA) ensemble is now known, comprises a growing almost seven decades ago (Cohn 1951), modi- number of chemical adducts: m6A, inosine (I), fied RNA bases were later also discovered to 5-methylcytidine (5mC), 5-hydroxymethylcyti- be abundant constituents of mRNA, with N6- dine (5hmC), pseudouridine (Ψ), 20-O-methyl- methyladenosine (m6A) as the most prevalent ation (Nm), and N1-methyladenosine (m1A). internal mRNA modification (Desrosiers et al. This review focuses on m6A as well as on more 1974). To date, 170 different modified nucleo- recently characterized mRNA modifications and tides have been identified in RNA from all types their effects on translation. and species (Boccaletto et al. 2018), expanding the RNA alphabet to embed transcripts with N6-METHYLADENOSINE (m6A) additional information. The introduction of transcriptome-wide mapping methods and the m6A is the most common internal (noncap) discovery of enzymes capable of removing m6A mRNA modification found in eukaryotic or- and of dedicated m6A-binding proteins have ganisms as well as in RNA of nuclear-replicat- Editors: Michael B. Mathews, Nahum Sonenberg, and John W.B. Hershey Additional Perspectives on Translation Mechanisms and Control available at www.cshperspectives.org Copyright © 2019 Cold Spring Harbor Laboratory Press; all rights reserved; doi: 10.1101/cshperspect.a032623 Cite this article as Cold Spring Harb Perspect Biol 2019;11:a032623 1 Downloaded from http://cshperspectives.cshlp.org/ on October 1, 2021 - Published by Cold Spring Harbor Laboratory Press E. Peer et al. ing viruses (reviewed in Fu et al. 2014). m6A ing family protein 1 (YTHDF1) and YTH do- constitutes 0.1%–0.5% of adenosine residues in main-containing family protein 3 (YTHDF3) mRNA, corresponding to ∼3–5m6A modifica- (regulation of translation); YTH domain-con- tions per transcript. Although m6A was discov- taining family protein 2 (YTHDF2) (regulation ered in mRNA decades ago (Desrosiers et al. of RNA turnover); and heterogeneous nuclear 1974), its study was jump-started in recent years ribonucleoprotein A2/B1 (HNRNPA2B1) and with the identification of the m6A-specific de- YTH domain-containing 1 (YTHDC1) (pro- methylases: the fat mass and obesity-associated cessing of primary microRNAs and alternative protein (FTO) and AlkB homolog 5 (ALKBH5) splicing) (Meyerand Jaffrey 2017). These discov- (Jia et al. 2011; Zheng et al. 2013), followed by eries opened up the field of epitranscriptomics the development of high-throughput m6A map- for accelerated investigation. ping tools, m6A-seq (Dominissini et al. 2012) and methylated RNA immunoprecipitation and PSEUDOURIDINE (Ψ) sequencing (MeRIP-Seq) (Meyer et al. 2012). Transcriptome-wide mapping of m6A revealed Ψ was the first posttranscriptional modification an evolutionarily conserved, nonrandom distri- to be identified and, taking all classes of RNA bution (Dominissini et al. 2012; Meyer et al. into account, is also the most abundant. Recent- 2012); m6A preferentially decorates 50 untrans- ly it was shown to be more abundant in mRNA lated regions (UTRs), long internal exons, and than previously believed, with a Ψ/U ratio the vicinity of stop codons. Sites preferentially of 0.2%–0.6% in mammalian mRNA (Li et al. appear within the consensus sequence RRACU 2015). It is formed by isomerization of uridine in (R = A or G) (Dominissini et al. 2012). which the base is rotated 180° along the N3-C6 The deposition of m6A in mRNA is per- axis. Although the modified base has unaltered formed by a multicomponent methyltransferase Watson–Crick base-pairing properties, it gains complex of which several components have been an additional hydrogen-bond donor at its non- identified: methyltransferase-like 3 (METTL3), Watson–Crick edge, thus endowing it with the catalytic component; methyltransferase- distinct chemical properties (Ge and Yu 2013). like 14 (METTL14), an RNA adaptor needed Ψ plays roles in the biogenesis and function for METTL3 activity; Wilms Tumor 1–associat- of spliceosomal small nuclear RNAs (snRNAs) ing protein (WTAP), a regulatory factor that and rRNA. Methods for transcriptome-wide is also responsible for nuclear speckle localiza- mapping of Ψ using Ψ-specific chemical labeling tion; RNA-binding motif (RBM) proteins 15/15B by CMC [N-cyclohexyl-N0-(2-morpholinoethyl) (RBM15/15B), mediators of methylation specif- carbodiimide metho-p-toluenesulfonate] iden- icity by directing the complex to specific tran- tified hundreds of Ψ sites in yeast and human scripts; and KIAA1429/Virilizer, whose func- mRNAs (Carlile et al. 2014; Schwartz et al. 2014; tion is still unclear (reviewed in Meyer and Li et al. 2015). No preferred localization of Ψ Jaffrey 2017). to specific regions of mRNA was shown. The balance between writing (adding) and Installation of Ψ is catalyzed by pseudo- erasing m6A from transcripts confers upon uridine synthases (PUSs) and can be achieved this modification a dynamic nature, which is ev- through two distinct mechanisms (Li et al. 2016a): ident under stress conditions (Dominissini et al. (1) RNA-independent pseudouridylation that is 2012), supporting a role in regulation of gene catalyzed by a single PUS enzyme responsible expression. Indeed, m6A plays a role in many for both substrate recognition and catalysis; or aspects of gene expression, affecting RNA splic- (2) RNA-dependent pseudouridylation that re- ing, nuclear export and retention, translation, lies on RNA–protein complexes consisting of and turnover. The major mechanism by which box H/ACA noncoding RNAs in conjunction m6A exerts its effects is by recruiting m6A reader with the centromere-binding factor 5 (Cbf5)/ proteins: fragile X mental retardation 1 (FMR1) Dyskerin protein complex. Although mRNA Ψ (Edupuganti et al. 2017); YTH domain-contain- is dynamically regulated in response to environ- 2 Cite this article as Cold Spring Harb Perspect Biol 2019;11:a032623 Downloaded from http://cshperspectives.cshlp.org/ on October 1, 2021 - Published by Cold Spring Harbor Laboratory Press The Epitranscriptome in Translation Regulation mental signals (Carlile et al. 2014), pseudouri- to a GUUCRA tRNA-like motif (R = A or G) dylation has not been shown to be reversible. and have T-loop-like structures, are evenly dis- tributed along transcript segments, and consti- fi N1-METHYLADENOSINE (m1A) tute roughly 10% of all identi ed sites. Last, TRMT61B-dependent m1A sites are primarily Using mass spectrometry and a mapping meth- located within the coding region of mitochon- odology based on immunocapture and massively drial mRNA, where they inhibit translation parallel sequencing, m1A was recently identified (Li et al. 2017b). as a new epitranscriptome marker that occurs on thousands of human and mouse mRNAs 20-O-METHYLATION (Nm) (Dominissini et al. 2016; Li et al. 2016b). m1A was known to exist in transfer RNA (tRNA) The ribose ring can be methylated at the 20 po- and rRNA and is unique in that it has both sition to form Nm (Boccaletto et al. 2018), a methyl group and a positive charge under an abundant modification present in mRNA, physiological conditions, which could markedly rRNA, tRNA, snRNA, and microRNA, which alter RNA structure and protein–RNA interac- is essential for their biogenesis, metabolism, tions. Transcriptome-wide mapping of m1A and function (Liang et al. 2009; Daffis et al. revealed unique features of its distribution that 2010; Lin et al. 2011; Züst et al. 2011; Deryu- strongly indicate functional roles: (1) m1Ais sheva et al. 2012; Jöckel et al. 2012; Somme et al. strongly enriched in mRNA 50UTRs, close to 2014). Nm modification is catalyzed either by translation initiation sites; (2) m1A is present in stand-alone methyltransferases that recognize highly structured regions