The race to decipher the top secrets of TOP mRNAs ☆ Oded Meyuhas a, ⁎, Tamar Kahan b a Department of Biochemistry and Molecular Biology, Institute for Medical Research — Israel-Canada, The Hebrew University-Hadassah Medical School, Jerusalem 91120, Israel b Bioinformatics Unit, The Hebrew University, Hadassah Medical School, Jerusalem 91120, Israel abstract article info Article history: Received 20 June 2014 Received in revised form 18 August 2014 Accepted 27 August 2014 Available online xxxx Keywords: TOP mRNAs S6K 4E-BP TIA-1 LARP1 miR-10a Cells encountering hostile growth conditions, like those residing in the middle of a newly developing solid tumor, conserve resources and energy by downregulating protein synthesis. One mechanism in this response is the translational repression of multiple mRNAs that encode components of the translational apparatus. This coordi- nated translational control is carried through a common cis-regulatory element, the 5′ Terminal OligoPyrimidine motif (5′TOP), after which these mRNAs are referred to as TOP mRNAs. Subsequent to the initial structural and functional characterization of members of this family, the research of TOP mRNAs has progressed in three major directions: a) delineating the landscape of the family; b) establishing the pathways that transduce stress cues into selective translational repression; and c) attempting to decipher the most proximal trans-acting factor(s) and defining its mode of action — a repressor or activator. The present chapter critically reviews the development in these three avenues of research with a special emphasis on the two “top secrets” of the TOP mRNA family: the scope of its members and the identity of the proximal cellular regulator(s). This article is part of a Special Issue entitled: Translation and Cancer. © 2014 Elsevier B.V. All rights reserved. 1. Introduction Members of the TOP mRNA family are characterized by several structural features: a) an invariable C residue at the cap site, followed by an uninterrupted stretch of 4 to 15 pyrimidines ([1,2]; b) a similar proportion of C and U residues within the pyrimidine stretch of most members; c) a CG-rich region immediately downstream of the 5′TOP motif; and d) a very high conservation of the 5′TOP motif and its adjacent transcribed sequence of a given member among mammals (Table 1). Moreover, the hallmarks of the 5′TOP motif are conserved among all vertebrates and extend even to ribosomal proteins mRNAs from Drosophila melanogaster [2] and to at least one TOP mRNA (encoding eukaryotic elongation factor 2, eEF2), from the mollusk Aplysia californica [3]. Nevertheless, neither the conservation of the motif nor the unique mode of regulation of the respective mRNAs has been reported, so far, for C. elegans and yeast. Initiation of transcription at a C residue is rare among eukaryotic genes, which normally start at a purine residue ([4] and references therein). Indeed, chemical analysis of the distribution of the cap structure among the 5′ terminal nucleotides in mouse mRNAs revealed that 73% of the cap is associated with A and G, 17% with C and 10% with U residue [5]. Strikingly, bioinformatics analysis of all 31,910 non-redundant transcription start site coordinates, according to the mRNA-RefGene annotation of human genome assembly hg38 by UCSC Genome Browser, disclosed a very similar distribution (70% at A and G, 19% at C and 11% at U residue). Evidently, the 5′TOP motif comprises the core of the translational cis-regulatory element of TOP mRNAs and its function is fully reliant on its integrity and location at the 5′ terminus, to the extent that it is abolished if the C at the cap site is replaced, or even just preceded by an A residue [6,7]. Moreover, full manifestation of the translational con- trol of TOP mRNAs, at least in some cell lines, appears to require both the 5′TOP motif and the CG-rich region [6]. Importantly, even in the presence of all hallmarks of the 5′TOP motif an mRNA is not necessarily subjected to a typical translational control. The followings are a few examples: a) The translational control of eEF2 mRNA seems to be confined to hematopoietic cells, unlike that of mRNAs encoding rps and eukaryotic elongation factor 1A (eEF1A), which are ubiquitously regulated [8]. Moreover, attempts to delineate the translational cis-regulatory element within TOP mRNAs have demonstrated that the first 27 to 35 nt of these mRNAs are sufficient to confer translational control on a reporter mRNA in a mitosis- dependent manner [6–9]. However, the first 29 nt of eEF2 mRNA can confer mitosis-dependent translational control on a reporter mRNA, even in cells where the endogenous mRNA is re- fractory to this mode of regulation [8]. This observation implies Biochimica et Biophysica Acta xxx (2014) xxx–xxx ☆ This article is part of a Special Issue entitled: Translation and Cancer. ⁎ Corresponding author. Tel.: +972 2 6758290; fax: +972 2 6757338. E-mail address: meyuhas@cc.huji.ac.il (O. Meyuhas). BBAGRM-00815; No. of pages: 11; 4C: 4, 6 http://dx.doi.org/10.1016/j.bbagrm.2014.08.015 1874-9399/© 2014 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Biochimica et Biophysica Acta journal homepage: www.elsevier.com/locate/bbagrm Please cite this article as: O. Meyuhas, T. Kahan, The race to decipher the top secrets of TOP mRNAs, Biochim. Biophys. Acta (2014), http:// dx.doi.org/10.1016/j.bbagrm.2014.08.015