* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download The role of IRES trans-acting factors in regulating translation initiation
RNA interference wikipedia , lookup
G protein–coupled receptor wikipedia , lookup
Eukaryotic transcription wikipedia , lookup
Gene regulatory network wikipedia , lookup
Cell-penetrating peptide wikipedia , lookup
Protein (nutrient) wikipedia , lookup
Bottromycin wikipedia , lookup
Silencer (genetics) wikipedia , lookup
Protein moonlighting wikipedia , lookup
RNA polymerase II holoenzyme wikipedia , lookup
Western blot wikipedia , lookup
Expression vector wikipedia , lookup
Nuclear magnetic resonance spectroscopy of proteins wikipedia , lookup
Paracrine signalling wikipedia , lookup
Polyadenylation wikipedia , lookup
Protein adsorption wikipedia , lookup
Transcriptional regulation wikipedia , lookup
Non-coding RNA wikipedia , lookup
Protein–protein interaction wikipedia , lookup
Proteolysis wikipedia , lookup
Two-hybrid screening wikipedia , lookup
List of types of proteins wikipedia , lookup
Gene expression wikipedia , lookup
Messenger RNA wikipedia , lookup
Post-Transcriptional Control: mRNA Translation, Localization and Turnover The role of IRES trans-acting factors in regulating translation initiation Helen A. King*†, Laura C. Cobbold† and Anne E. Willis†1 *School of Pharmacy, Centre for Biomolecular Science, University Park, University of Nottingham, Nottingham NG7 2RD, U.K., and †MRC Toxicology Unit, Hodgkin Building, Lancaster road, University of Leicester, Leicester LE1 9HN, U.K. Abstract The majority of mRNAs in eukaryotic cells are translated via a method that is dependent upon the recognition of, and binding to, the methylguanosine cap at the 5’ end of the mRNA, by a set of protein factors termed eIFs (eukaryotic initiation factors). However, many of the eIFs involved in this process are modified and become less active under a number of pathophysiological stress conditions, including amino acid starvation, heat shock, hypoxia and apoptosis. During these conditions, the continued synthesis of proteins essential to recovery from stress or maintenance of a cellular programme is mediated via an alternative form of translation initiation termed IRES (internal ribosome entry site)-mediated translation. This relies on the mRNA containing a complex cis-acting structural element in its 5’-UTR (untranslated region) that is able to recruit the ribosome independently of the cap, and is often dependent upon additional factors termed ITAFs (IRES trans-acting factors). A limited number of ITAFs have been identified to date, particularly for cellular IRESs, and it is not yet fully understood how they exert their control and which cellular pathways are involved in their regulation. Cap-dependent translation initiation Translation of mRNA into protein involves three stages: initiation, elongation and termination. The initiation step is tightly regulated to allow the cell to respond efficiently to a given stimulus and is the rate-limiting step. Capdependent initiation relies upon recognition of the m7 GpppN (7-methylguanosine) cap at the 5 end of the mRNA by a complex of canonical initiation factors (Figure 1). eIF (eukaryotic initiation factor) 4F, which comprises eIF4A (a DEADbox RNA helicase), eIF4E (the cap-binding protein), and eIF4G (a multi-domain scaffold protein), recognizes and binds the 5 -cap. This is then able to recruit the 40S ribosomal subunit along with ternary complex (GTP-bound eIF2 and charged methionine initiator-tRNA) as part of the 43S initiation complex, via interactions between eIF3 (a large multi-subunit protein) and eIF4G (Figure 1). Once assembled, the complex is referred to as the 48S initiation complex: this scans along the 5 -UTR (untranslated region) of the mRNA with the helicase eIF4A assisting in resolution of any complex and potentially inhibitory secondary structure. Scanning continues until an AUG in optimal ‘Kozak’ consensus context is encountered [1]. Release of the initiation factors is triggered by eIF5-mediated hydrolysis of the GTP bound to eIF2. This disassembly of the complex Key words: eukaryotic initiation factor (eIF), internal ribosome entry site (IRES), IRES trans-acting factor (ITAF), translation initiation, untranslated region (UTR). Abbreviations used: Apaf-1, apoptotic peptidase-activating factor 1; Bag-1, Bcl-2-associated athanogene-1; BiP, immunoglobulin heavy-chain-binding protein; eIF, eukaryotic initiation factor; 4E-BP, eIF4E-binding protein; EMCV, encephalomyocarditis virus; IRES, internal ribosome entry site/segment; ITAF, IRES trans-acting factor; PABP, poly(A)-binding protein; PTB, polypyrimidinetract-binding protein; Unr, upstream of N-Ras; UTR, untranslated region. 1 To whom correspondence should be addressed (email [email protected]). Biochem. Soc. Trans. (2010) 38, 1581–1586; doi:10.1042/BST0381581 allows the 60S ribosomal subunit to join to the 40S and form the complete 80S ribosome. The ribosome then enters the elongation phase of translation. (For detailed reviews of the mechanism of cap-dependent translation initiation, see [2–4].) Several features in the 5 -UTR of an mRNA are inhibitory to the progress of the scanning ribosome, including uAUGs (upstream AUGs), a long length of UTR and a high degree of secondary structure. However, a number of cap-dependent mechanisms exist to overcome the effects of these inhibitory elements, including ribosomal shunting, leaky scanning and termination-reinitiation [5,6]. There are a number of situations in which cap-dependent initiation is compromised, due either to cleavage of one or more of the canonical initiation factors including eIF4G, eIF4b and eIF3 [7], or a change in the phosphorylation state of the factors and their binding partners. During poliovirus infection, viral protease 2A is able to cleave eIF4G at Arg486 -Gly487 , which separates the eIF4Ebinding site on eIF4G from the eIF4A- and eIF3-binding sites. This abolishes the eIF4E-binding function of eIF4G and, as a consequence, inhibits cap-dependent translation [8]. Interestingly, a similar situation occurs during apoptosis, where eIF4G is cleaved by effector caspases into three fragments termed C-, M- and N-FAG (C-terminal, middle and N-terminal fragment respectively), which separates the PABP [poly(A)-binding protein] binding site from the eIF4A-, eIF3- and eIF4E-binding sites [9]. Cap-dependent translation can also be inhibited by phosphorylation of eIF2 on the α subunit, or hypophosphorylation of 4E-BP (eIF4E-binding protein). Phosphorylation of eIF2 occurs during many stress conditions, including amino acid starvation and hypoxia, as C The C 2010 Biochemical Society Authors Journal compilation 1581 1582 Biochemical Society Transactions (2010) Volume 38, part 6 Figure 1 Cap-dependent translation initiation complex Figure 2 Methods of cap-dependent translation inhibition The 5 -cap is recognized by eIF4E as part of the eIF4F complex, which in turn recruits the 43S initiation complex including the 40S ribosomal (a) Hypophosphorylation of 4E-BP due to serum starvation or picornavirus infection allows it to compete for binding to eIF4E with eIF4G, subunit to form the 48S initiation complex at the cap (see the text for details). The mRNA is circularized via interactions between eIF4G at the 5 -end and PABP at the 3 -end. The 48S complex scans through causing eIF4F levels to become limiting. When hyperphosphorylated, for example during growth conditions, 4E–BP is no longer able to bind eIF4E. (b) Phosphorylation of the α subunit of eIF2 by kinases such as the 5 -UTR until an AUG in optimal context is encountered, where the complex stalls, the initiation factors dissociate, and the 60S ribosomal subunit joins to form the translation elongation competent 80S ribosome. PKR (double-stranded-RNA-dependent protein kinase) cause it to bind with stronger affinity to its guanine-nucleotide-exchange factor eIF2B, resulting in levels of ternary complex becoming limiting. Meti , initiator methionine. well as during stages of the standard cell cycle, such as mitosis. Phosphorylation of eIF2α at Ser51 increases its affinity for its guanine-nucleotide-exchange factor eIF2B, so that it remains associated with eIF2B and is no longer recycled into a new ternary complex. In this way, the amount of ternary complex becomes limiting for cap-dependent initiation (Figure 2b) [5]. Hypophosphorylation of 4E-BP occurs during heat shock, serum starvation and picornavirus infection. When hyperphosphorylated, 4E-BP cannot bind eIF4E. However, when hypophosphorylated, 4E-BP competes for binding to eIF4E with eIF4G, and consequently sequesters eIF4E away from the eIF4F complex. This results in levels of eIF4F becoming limiting and cap-dependent translation being inhibited (Figure 2a) [5]. IRES (internal ribosome entry site)-mediated translation The first IRESs were originally identified in members of the picornaviridae [10,11]. This virus family have positivesense RNA genomes which are uncapped, yet are translated efficiently in eukaryotic cells, therefore it was hypothesized that they may be translated via a cap-independent method. Subsequently it was shown that the 5 -UTRs of EMCV (encephalomyocarditis virus) and poliovirus are able to drive translation initiation by recruiting the ribosomal subunit directly, independent of active eIF4F [10,11] (Figure 3a). Eukaryotic IRESs were later identified, the first in the mRNA encoding BiP (immunoglobulin heavy-chain-binding protein) [12]. It was observed that translation of BiP is maintained during poliovirus infection, when several of the canonical initiation factors are cleaved as discussed above. Bicistronic IRES vectors were used to demonstrate a functional IRES in the BiP 5 -UTR, where expression of the C The C 2010 Biochemical Society Authors Journal compilation first cistron is under 5 -cap-dependent translational control, and the second cistron is under IRES-dependent control [12]. The bicistronic vector assay has received some criticism, due to the possibility of apparent IRES-mediated expression of the second cistron arising from aberrant splicing, the presence of a cryptic promoter or ribosomal read-through from the first cistron [13]. However, with the correct control experiments in place, these possibilities can be reliably ruled out, as demonstrated in [14]. Since these initial discoveries, the list of mRNAs known to contain IRES elements, and hence have the ability to utilize cap-independent translation, has been growing steadily, and in silico analyses estimate that up to 10 % of cellular mRNAs may contain an IRES element [15]. Importantly, the protein products of IRES-containing mRNAs tend to be involved in control of cell growth or cell death [16]. The presence of an IRES element in such mRNAs allows their translation to be either maintained or up-regulated under conditions where cap-dependent translation is inhibited. Furthermore, they are known to require a combination of both specific canonical initiation factors and auxiliary trans-acting factors for their function. ITAFs (IRES trans-acting factors) The activity of a given IRES can vary greatly between different cell lines, due to variation in the availability of ITAFs. For example, it was found that translation mediated by the IRES of both poliovirus and rhinovirus is weak in the Post-Transcriptional Control: mRNA Translation, Localization and Turnover Figure 3 IRES-mediated translation initiation (a) An IRES is a complex structural element within the 5 -UTR that is able to recruit the 43S initiation complex in a cap-independent manner. Specific proteins termed ITAFs are required for this recruitment. (b) Bag-1 mRNA contains an IRES which has been shown to require both PCBP1 and PTB for function. Initially, the protein PCBP1 binds to a stem–loop in the IRES, which modifies the loop such that two PTB proteins can bind, which in turn allows recruitment of the 40S ribosomal subunit. In this example, the ITAFs are behaving as RNA chaperones. Adapted from [32] with c American Society for Microbiology, Molecular and Cellular Biology, vol. 24, 2004, pp. 5595–5605, permission. Copyright doi:10.1128/MCB.24.12.5595-5605.2004. rabbit reticulocyte lysate system; however, their activity is restored by addition of a ribosome salt wash from HeLa cells [17]. This cell-type variability is also true for cellular IRES: the activity of the c-myc IRES was tested in a wide range of cell lines, and was found to be 20-fold more active in HeLa cells than in MCF7 cells, owing to the differing availability of ITAFs between these two cell lines [18]. The mechanism of action of ITAFs is not fully understood, but it is thought that they act either as RNA chaperones, changing or stabilizing the secondary structure of the IRES to allow further proteins or the 40S ribosomal subunit to bind (Figure 3b), or as adaptor proteins, acting as anchors to which other proteins or the 40S ribosomal subunit could bind [16]. Both cellular and viral IRESs require the action of ITAFs, and indeed many compete for the same proteins. The ITAF requirements of numerous IRESs have been examined (Table 1, and for a more comprehensive list, see [19]). The ITAF that has been most extensively studied is the multi- functional RNA-binding protein PTB (polypyrimidinetract-binding protein). In addition to a role in translation, this protein is also involved in mRNA splicing, stability and localization within the cell [20]. The data suggest that the majority of cellular IRESs require PTB for function, and PTB was shown to be particularly important for the control of IRESs which are active during apoptosis [21,22]. The regulation that an ITAF confers upon an IRES may be either a positive or a negative one. For example, whereas PTB regulates both p27Kip1 and BiP IRES, its binding enhances p27Kip1 IRES activity, but down-regulates BiP IRES activity [23–25]. Interestingly, the mRNAs that encode ITAFs have also been shown to contain IRESs. For example, Unr (upstream of N-Ras), an ITAF known to positively regulate several IRESs, is encoded by an mRNA that itself contains an IRES which is negatively regulated by Unr protein in an auto-regulatory negative-feedback loop, as well as by PTB [26–28]. C The C 2010 Biochemical Society Authors Journal compilation 1583 1584 Biochemical Society Transactions (2010) Volume 38, part 6 Table 1 Examples of known trans-acting factors, and the IRESs which they regulate The list of known IRESs and the ITAFs which regulate them has been growing since IRESs were first identified. ITAFs for both viral and cellular IRESs are listed. There are many other ITAFs and IRESs which have not been included owing to space restrictions. For a more comprehensive list of IRESs, see http://www.iresite.org/ [19]. FMDV, foot-and-mouth disease virus; HAV, hepatitis A virus; HCV, hepatitis C virus; hnRNP, heterogeneous nuclear ribonucleoprotein; HRV, human rhinovirus; IGF-1R, insulin-like growth factor 1 receptor; PCBP, poly(rC)-binding protein; PDGF2, platelet-derived growth factor 2; PV1, poliovirus 1; TMEV, Theiler’s murine encephalomyelitis virus; XIAP, Table 1 (Continued) ITAF/trans-acting factor IRES Reference HIV1 [56] BiP EMCV [57] [58] HCV XIAP Coxsackievirus B3 [59] [54] [51] HCV [60] X-linked inhibitor of apoptosis; YB1, Y-box-binding protein 1. ITAF/trans-acting factor IRES Reference PTB (hnRNPI) EMCV FMDV TMEV [36] [37] [38] PV1 HAV BiP [39] [40] [24] Apaf-1 Bag-1 [41] [41] IGF-1R p27kip1 Unr [42] [23] [28] Mnt Myb p53 [22] [22] [43] Cytochrome P450 (CYP1b1) SetD7 Cyclin T1 [21] [21] [21] Cat-1 TMEV Apaf-1 [44] [45] [46] Unr HRV Apaf-1 Unr [47] [46] [26] ITAF45 hnRNPA1 FMDV Cyclin D1 [38] [48] hnRNPE2 (PCBP2) c-myc PV1 Coxsackievirus B3 [49] [50] [51] c-myc HRV PV1 [49] [52] [50] c-myc PDGF2/c-sis XIAP [49] [53] [54] c-myc Cat-1 c-myc [49] [44] [34] GRSF YB1 c-myc c-myc c-myc [34] [34] [34] La PV1 [55] nPTB (neural PTB) hnRNPE1 (PCBP1) hnRNPC1/C2 hnRNPL PSF p54nrb C The C 2010 Biochemical Society Authors Journal compilation Significant advances have been made in identifying how ITAFs regulate viral IRESs. For example, it is known that PTB regulates the EMCV IRES by stabilizing its threedimensional structure, thus acting as an RNA chaperone [29]. Moreover, research in the viral IRES field has been aided by structural similarities between the IRESs of viruses that harbour different primary sequences. One such example is the presence of a large stem–loop structure at the 3 -end of the 5 -UTR in both the HCV (hepatitis C virus) and CSFV (classical swine fever virus) IRESs, which acts as the ribosome landing site in both cases [30,31]. For the cellular IRESs identified and studied to date, there has been no structural or sequence parallels observed, making it challenging to extrapolate findings from one IRES to the next [31]. However, a few interactions between cellular IRESs and their cognate ITAFs have been defined. In particular, the Apaf-1 (apoptotic peptidase-activating factor 1) and Bag-1 (Bcl-2-associated athanogene-1) IRESs have been extensively studied in this regard, and the data suggest that ITAFs remodel the structures of these two IRESs so that they attain the correct conformation for interaction with the 40S ribosomal subunit [27,32]. Thus the Bag-1 IRES requires the ITAF PCBP1 [poly(rC)-binding protein 1] (as an RNA chaperone) to bind to domain II and open up an adjacent structure in domain III, and then PTB (again as an RNA chaperone) to bind and expose the ribosome landing site [32] (Figure 3b). Likewise, the Apaf-1 IRES requires Unr to bind first and open up the secondary structure, which then allows two nPTB (neural PTB) molecules to bind, which in turn exposes the ribosome landing site [27]. Similarly, the secondary structure of the XIAP (X-linked inhibitor of apoptosis) IRES has been predicted and the IRES is known to require the ITAFs PTB, La and hnRNP (heterogeneous nuclear ribonucleoprotein) C1/C2 for function [33]. Finally, it has been shown that mutations in cellular IRESs can affect their interaction with ITAFs and alter their activity. For example, in multiple myeloma-derived cell lines and in patients with this disease, the c-myc IRES was found to contain a single C>T base substitution. This mutation results in an increase in IRES-mediated translation of c-myc and hence a large increase in the amount of c-myc protein. Thus dysregulated cell proliferation was shown to be due to an increased affinity of two ITAFs, PTB and YB1 (Y-boxbinding protein 1), for the mutated IRES [34,35]. By reducing Post-Transcriptional Control: mRNA Translation, Localization and Turnover the levels of these ITAFs, it was shown that it was possible to reduce the expression of c-myc and cell proliferation [34,35]. Several other ITAFs required by c-myc have also been identified and are shown in Table 1. Although work has been carried out on individual IRESs, there has as yet been no large-scale screen of IRESs and their trans-acting factor requirements. Overview and perspectives Cap-dependent translation initiation is the main mechanism by which translation-competent ribosomes are recruited to cellular mRNAs. However, this mechanism is not able to function under certain normal cellular conditions, such as during mitosis, as well as during physiological stress conditions, owing to a number of control mechanisms which modify the canonical initiation factors, thereby reducing their availability and/or activity. During these times, the cell must still be able to produce protein in order to overcome the period of stress or cell cycle and either emerge intact or undergo programmed cell death. Many proteins that are essential to these outcomes contain an IRES in the 5 -UTR of their respective mRNAs. IRESs have different requirements for both the canonical initiation factors and auxiliary factors termed ITAFs. The levels of these ITAFs vary between cell lines, explaining the cell-line-dependence of IRESs in terms of activity. Finetuning of ITAF levels is likely to allow the cell exquisite control over the activity of different IRESs. Work is being undertaken to identify and classify ITAFs in order to allow control over IRES-mediated protein expression. In particular, in our laboratory, we are interested in the IRESs that are active during apoptosis, a cellular process subverted in many diseases. Control over IRESmediated expression would potentially allow us to dictate cell fate, which has far-reaching implications for many diseases including cancers and neurological diseases. Funding A.E.W. holds a Professorial Fellowship from the Biotechnology and Biological Sciences Research Council. H.A.K. and L.C.C. are funded by a grant from the Biotechnology and Biological Sciences Research Council. References 1 Kozak, M. (1987) An analysis of 5 -noncoding sequences from 699 vertebrate messenger RNAs. Nucleic Acids Res. 15, 8125–8148 2 Pestova, T.V., Kolupaeva, V.G., Lomakin, I.B., Pilipenko, E.V., Shatsky, I.N., Agol, V.I. and Hellen, C.U.T. (2001) Molecular mechanisms of translation initiation in eukaryotes. Proc. Natl. Acad. Sci. U.S.A. 98, 7029–7036 3 Sachs, A.B. and Varani, G. (2000) Eukaryotic translation initiation: there are (at least) two sides to every story. Nat. Struct. Biol. 7, 356–361 4 Gingras, A.C., Raught, B. and Sonenberg, N. (1999) eIF4 initiation factors: effectors of messenger RNA recruitment to ribosomes and regulators of translation. Annu. Rev. Biochem. 68, 913–963 5 Pain, V.M. (1996) Initiation of protein synthesis in eukaryotic cells. Eur. J. Biochem. 236, 747–771 6 López-Lastra, M., Rivas, A. and Barrı́a, M.I. (2005) Protein synthesis in eukaryotes: the growing biological relevance of cap-independent translation initiation. Biol. Res. 38, 121–146 7 Bushell, M., Wood, W., Carpenter, G., Pain, V.M., Morley, S.J. and Clemens, M.J. (2001) Disruption of the interaction of mammalian protein synthesis eukaryotic initiation factor 4B with the poly(A)-binding protein by caspase- and viral protease-mediated cleavages. J. Biol. Chem. 276, 23922–23928 8 Marash, L. and Kimchi, A. (2005) DAP5 and IRES-mediated translation during programmed cell death. Cell Death Differ. 12, 554–562 9 Bushell, M., Wood, W., Clemens, M.J. and Morley, S.J. (2000) Changes in integrity and association of eukaryotic protein synthesis initiation factors during apoptosis. Eur. J. Biochem. 267, 1083–1091 10 Jang, S.K., Kräusslich, H.-G., Nicklin, M.J.H., Duke, G.M., Palmenberg, A.C. and Wimmer, E. (1988) A segment of the 5 nontranslated region of encephalomyocarditis virus RNA directs internal entry of ribosomes during in vitro translation. J. Virol. 62, 2636–2643 11 Pelletier, J. and Sonenberg, N. (1988) Internal initiation of translation of eukaryotic mRNA directed by a sequence derived from poliovirus RNA. Nature 33, 320–325 12 Macejak, D.G. and Sarnow, P. (1991) Internal initiation of translation mediated by the 5 leader of a cellular mRNA. Nature 353, 90–94 13 Kozak, M. (2005) A second look at cellular mRNA sequences said to function as internal ribosome entry sites. Nucleic Acids Res. 33, 6593–6602 14 Spriggs, K.A., Cobbold, L.C., Ridley, S.H., Coldwell, M.J., Bottley, A., Bushell, M., Willis, A.E. and Siddle, K. (2009) The human insulin receptor mRNA contains a functional internal ribosome entry segment. Nucleic Acids Res. 37, 5881–5893 15 Spriggs, K.A., Stoneley, M., Bushell, M. and Willis, A.E. (2008) Re-programming of translation following cell stress allows IRES-mediated translation to predominate. Biol. Cell 100, 27–38 16 Stoneley, M. and Willis, A.E. (2004) Cellular internal ribosome entry segments: structures, trans-acting factors and regulation of gene expression. Oncogene 23, 3200–3207 17 Brown, B.A. and Ehrenfeld, E. (1979) Translation of poliovirus RNA in vitro: changes in cleavage pattern and initiation sites by ribosomal salt wash. Virology 97, 396 18 Stoneley, M., Subkhankulova, T., Le Quesne, J.P., Coldwell, M.J., Jopling, C.L., Belsham, G.J. and Willis, A.E. (2000) Analysis of the c-myc IRES: a potential role for cell-type specific trans-acting factors and the nuclear compartment. Nucleic Acids Res. 28, 687–694 19 Mokrejš, M., Mašek, T., Vopálenský, V., Hlubuček, P., Delbos, P. and Pospı́šek, M. (2010) IRESite: a tool for the examination of viral and cellular internal ribosome entry sites. Nucleic Acids Res. 38, D131–136 20 Sawicka, K., Bushell, M., Spriggs, K.A. and Willis, A.E. (2008) Polypyrimidine-tract-binding protein: a multifunctional RNA-binding protein. Biochem. Soc. Trans. 36, 641–647 21 Bushell, M., Stoneley, M., Kong, Y.W., Hamilton, T.L., Spriggs, K.A., Dobbyn, H.C., Qin, X., Sarnow, P. and Willis, A.E. (2006) Polypyrimidine tract binding protein regulates IRES-mediated gene expression during apoptosis. Mol. Cell 23, 401–412 22 Mitchell, S.A., Spriggs, K.A., Bushell, M., Evans, J.R., Stoneley, M., Le Quesne, J.P.C., Spriggs, R.V. and Willis, A.E. (2005) Identification of a motif that mediates polypyrimidine tract-binding protein-dependent internal ribosome entry. Genes Dev. 19, 1556–1571 23 Cho, S., Kim, J.H., Back, S.H. and Jang, S.K. (2005) Polypyrimidine tract-binding protein enhances the internal ribosomal entry site-dependent translation of p27kip1 mRNA and modulates transition from G1 to S phase. Mol. Cell. Biol. 25, 1283–1297 24 Kim, Y.K., Hahm, B. and Jang, S.K. (2000) Polypyrimidine tract-binding protein inhibits translation of BiP mRNA. J. Mol. Biol. 304, 119–133 25 Jiang, H., Coleman, J., Miskimins, R., Srinivasan, R. and Miskimins, K.W. (2007) Cap-independent translation through the p27 5 UTR. Nucleic Acids Res. 35, 4767–4778 26 Dormoy-Raclet, V., Markovits, J., Jacquemin-Sablon, A. and Jacquemin-Sablon, H. (2005) Regulation of Unr expression by 5 - and 3 -untranslated regions of its mRNA through modulation of stability and IRES mediated translation. RNA Biol. 2, e27–e35 27 Mitchell, S.A., Spriggs, K.A., Coldwell, M.J., Jackson, R.J. and Willis, A.E. (2003) The Apaf-1 internal ribosome entry segment attains the correct structural conformation for function via interactions with PTB and unr. Mol. Cell 11, 757–771 28 Cornelis, S., Tinton, S.A., Schepens, B., Bruynooghe, Y. and Beyaert, R. (2005) UNR translation can be driven by an IRES element that is negatively regulated by polypyrimidine tract binding protein. Nucleic Acids Res. 33, 3095–3108 C The C 2010 Biochemical Society Authors Journal compilation 1585 1586 Biochemical Society Transactions (2010) Volume 38, part 6 29 Kafasla, P., Morgner, N., Pöyry, T.A.A., Curry, S., Robinson, C.V. and Jackson, R.J. (2009) Polypyrimidine tract binding protein stabilizes the encephalomyocarditis virus IRES structure via binding multiple sites in a unique orientation. Mol. Cell 34, 556–568 30 Brown, E.A., Zhang, H., Ping, L.H. and Lemon, S.M. (1992) Secondary structure of the 5 nontranslated regions of hepatitis C virus and pestivirus genomic RNAs. Nucleic Acids Res. 20, 5041–5045 31 Baird, S.D., Lewis, S.M., Turcotte, M. and Holcik, M. (2007) A search for structurally similar cellular internal ribosome entry sites. Nucleic Acids Res. 35, 4664–4677 32 Pickering, B.M., Mitchell, S.A., Spriggs, K.A., Stoneley, M. and Willis, A.E. (2004) Bag-1 internal ribosome entry segment activity is promoted by structural changes mediated by the poly(rC) binding protein 1 and recruitment of polypyrimidine tract binding protein 1. Mol. Cell. Biol. 24, 5595–5605 33 Holcik, M., Gordon, B.W. and Korneluk, R.G. (2003) The internal ribosome entry site-mediated translation of antiapoptotic protein XIAP is modulated by the heterogeneous nuclear ribonucleoproteins C1 and C2. Mol. Cell. Biol. 23, 280–288 34 Cobbold, L.C., Spriggs, K.A., Haines, S.J., Dobbyn, H.C., Hayes, C., de Moor, C.H., Lilley, K.S., Bushell, M. and Willis, A.E. (2008) Identification of internal ribosome entry segment (IRES)-trans-acting factors for the Myc family of IRESs. Mol. Cell. Biol. 28, 40–49 35 Cobbold, L.C., Wilson, L.A., Sawicka, K., King, H.A., Kondrashov, A.V., Spriggs, K.A., Bushell, M. and Willis, A.E. (2010) Upregulated c-myc expression in multiple myeloma by internal ribosome entry results from increased expression of PTB-1 and YB-1. Oncogene 29, 2884–2891 36 Pestova, T.V., Shatsky, I.N. and Hellen, C.U. (1996) Functional dissection of eukaryotic initiation factor 4F: the 4A subunit and the central domain of the 4G subunit are sufficient to mediate internal entry of 43S preinitiation complexes. Mol. Cell. Biol. 16, 6870–6878 37 Niepmann, M., Petersen, A., Meyer, K. and Beck, E. (1997) Functional involvement of polypyrimidine tract-binding protein in translation initiation complexes with the internal ribosome entry site of foot-and-mouth disease virus. J. Virol. 71, 8330–8339 38 Pilipenko, E.V., Pestova, T.V., Kolupaeva, V.G., Khitrina, E.V., Poperechnaya, A.N., Agol, V.I. and Hellen, C.U. (2000) A cell cycle-dependent protein serves as a template-specific translation initiation factor. Genes Dev. 14, 2028–2045 39 Toyoda, H., Koide, N., Kamiyama, M., Tobita, K., Mizumoto, K. and Imura, N. (1994) Host factors required for internal initiation of translation on poliovirus RNA. Arch. Virol. 138, 1–15 40 Gosert, R., Chang, K.H., Rijnbrand, R., Yi, M., Sangar, D.V. and Lemon, S.M. (2000) Transient expression of cellular polypyrimidine-tract binding protein stimulates cap-independent translation directed by both picornaviral and flaviviral internal ribosome entry sites in vivo. Mol. Cell. Biol. 20, 1583–1595 41 Coldwell, M.J., deSchoolmeester, M., Fraser, G.A., Pickering, B.M., Packham, G. and Willis, A.E. (2001) The p36 isoform of BAG-1 is translated by internal ribosome entry following heat shock. Oncogene 20, 4095–5000 42 Giraud, S., Greco, A., Brink, M., Diaz, J.J. and Delafontaine, P. (2001) Translation initiation of the insulin-like growth factor I receptor mRNA is mediated by an internal ribosome entry site. J. Biol. Chem. 276, 5668–5675 43 Ray, P.S., Grover, R. and Das, S. (2006) Two internal ribosome entry sites mediate the translation of p53 isoforms. EMBO Rep. 7, 404–410 44 Majumder, M., Yaman, I., Gaccioli, F., Zeenko, V.V., Wang, C., Caprara, M.G., Venema, R.C., Komar, A.A., Snider, M.D. and Hatzoglou, M. (2009) The hnRNA-binding proteins hnRNP L and PTB are required for efficient translation of the Cat-1 arginine/lysine transporter mRNA during amino acid starvation. Mol. Cell. Biol. 29, 2899–2912 C The C 2010 Biochemical Society Authors Journal compilation 45 Pilipenko, E.V., Viktorova, E.G., Guest, S.T., Agol, V.I. and Roos, R.P. (2001) Cell-specific proteins regulate viral RNA translation and virus-induced disease. EMBO J. 20, 6899–6908 46 Mitchell, S.A., Brown, E.C., Coldwell, M.J., Jackson, R.J. and Willis, A.E. (2001) Protein factor requirements of the Apaf-1 internal ribosome entry segment: roles of polypyrimidine tract-binding protein and upstream of N-ras. Mol. Cell. Biol. 21, 3364–3374 47 Hunt, S.L., Hsuan, J.J., Totty, N. and Jackson, R.J. (1999) unr, a cellular cytoplasmic RNA-binding protein with five cold-shock domains, is required for internal initiation of translation of human rhinovirus RNA. Genes Dev. 13, 437–448 48 Jo, O.D., Martin, J., Masri, J., Lichtenstein, A. and Gera, J. (2008) Heterogeneous nuclear ribonucleoprotein A1 regulates cyclin D1 and c-myc IRES function through Akt signaling. J. Biol. Chem. 283, 23274–23287 49 Evans, J.R., Mitchell, S.A., Spriggs, K.A., Ostrowski, J., Bomsztyk, K., Ostarek, D. and Willis, A.E. (2003) Members of the poly(rC) binding protein family stimulate the activity of the c-myc internal ribosome entry segment in vitro and in vivo. Oncogene 22, 8012–8021 50 Blyn, L.B., Towner, J.S., Semler, B.L. and Ehrenfeld, E. (1997) Requirement of poly(rC) binding protein 2 for translation of poliovirus RNA. J. Virol. 71, 6243–6246 51 Ray, P.S. and Das, S. (2002) La autoantigen is required for the internal ribosome entry site-mediated translation of Coxsackievirus B3 RNA. Nucleic Acids Res. 30, 4500–4508 52 Choi, K., Kim, J.H., Li, X., Paek, H.Y., Ha, S.H., Ryu, S.H., Wimmer, E. and Jang, S.K. (2004) Identification of cellular proteins enhancing activities of internal ribosomal entry sites by competition with oligodeoxynucleotides. Nucleic Acids Res. 32, 1308–1317 53 Bernstein, J., Sella, O., Le, S.Y. and Elroy-Stein, O. (1997) PDGF2/c-sis mRNA leader contains a differentiation-linked internal ribosomal entry site (D-IRES). J. Biol. Chem. 272, 9356–9362 54 Holcik, M. and Korneluk, R.G. (2000) Functional characterization of the X-linked inhibitor of apoptosis (XIAP) internal ribosome entry site element: role of La autoantigen in XIAP translation. Mol. Cell. Biol. 20, 4648–4657 55 Meerovitch, K., Pelletier, J. and Sonenberg, N. (1989) A cellular protein that binds to the 5 -noncoding region of poliovirus RNA: implications for internal translation initiation. Genes Dev. 3, 1026–1034 56 Chang, Y.N., Kenan, D.J., Keene, J.D., Gatignol, A. and Jeang, K.T. (1994) Direct interactions between autoantigen La and human immunodeficiency virus leader RNA. J. Virol. 68, 7008–7020 57 Kim, J.G., Armstrong, R.C., Berndt, J.A., Kim, N.W. and Hudson, L.D. (1998) A secreted DNA-binding protein that is translated through an internal ribosome entry site (IRES) and distributed in a discrete pattern in the central nervous system. Mol. Cell. Neurosci. 12, 119–140 58 Kim, Y.K. and Jang, S.K. (1999) La protein is required for efficient translation driven by encephalomyocarditis virus internal ribosomal entry site. J. Gen. Virol. 80, 3159–3166 59 Isoyama, T., Kamoshita, N., Yasui, K., Iwai, A., Shiroki, K., Toyoda, H., Yamada, A., Takasaki, Y. and Nomoto, A. (1999) Lower concentration of La protein is required for internal ribosome entry on hepatitus C virus RNA than on poliovirus RNA. J. Gen. Virol. 80, 2319–2327 60 Cordes, S., Kusov, Y., Heise, T. and Gauss-Müller, V. (2008) La autoantigen suppresses IRES-dependent translation of the hepatitis A virus. Biochem. Biophys. Res. Commun. 368, 1014–1019 Received 7 June 2010 doi:10.1042/BST0381581