Identification of Internal Ribosome Entry Segment (IRES)-trans-Acting Factors for the Myc Family of IRESs

Abstract

The proto-oncogenes c-, L-, and N-myc can all be translated by the alternative method of internal ribosome entry whereby the ribosome is recruited to a complex structural element (an internal ribosome entry segment [IRES]). Ribosome recruitment is dependent upon the presence of IRES-trans-acting factors (ITAFs) that act as RNA chaperones and allow the mRNA to attain the correct conformation for the interaction of the 40S subunit. One of the major challenges for researchers in this area is to determine whether there are groups of ITAFs that regulate the IRES-mediated translation of subsets of mRNAs. We have identified four proteins, termed GRSF-1 (G-rich RNA sequence binding factor 1), YB-1 (Y-box binding protein 1), PSF (polypyrimidine tract binding protein-associated splicing factor), and its binding partner, p54nrb, that bind to the myc family of IRESs. We show that these proteins positively regulate the translation of the Myc family of oncoproteins (c-, L-, and N-Myc) in vivo and in vitro. Interestingly, synthesis from the unrelated IRESs, BAG-1 and Apaf-1, was not affected by YB-1, GRSF-1, or PSF levels in vivo, suggesting that these three ITAFs are specific to the myc IRESs. Myc proteins play a role in cell proliferation; therefore, these results have important implications regarding the control of tumorigenesis.The proteins encoded by the myc gene family function as sequence-specific transcription factors that regulate target genes integral to the processes of cell proliferation, differentiation, and cell death (2, 9). Although many of the functions of the three major myc genes are overlapping, unique properties have been ascribed to individual Myc proteins. For example, each of the Myc proteins can restore the growth and proliferative defects of c-myc null fibroblasts and can promote apoptosis following growth factor deprivation (23,30). In addition, all three proteins can regulate known myc target genes (23). Unique roles for these proteins are supported by the following observations: during embryogenesis and in adult tissues, the expression patterns of c-, L-, and N-Myc are distinct (52); homozygous null c-or N-myc mice die early in development, whereas targeted disruption of both L-myc alleles is not lethal; and in some instances, L-myc displays distinct cell transformation and transcriptional properties (1, 38, 39).It is not surprising, given the role of the Myc family of proteins in proliferation and apoptosis, that the expression of these proteins is highly regulated at the levels of both transcription and translation. Indeed, deregulated Myc expression, through either of these mechanisms, has been associated with tumorigenesis (12,35,36,49,50).Previous studies have shown that the 5Ј untranslated regions (UTRs) of c-, L-, and N-myc encoded by exon 1 each contain a complex RNA structural element known as an internal ribosome entry segment (IRES). Consequently, synthesis of the Myc family proteins can occur by the process of internal ribosome entry (18,19,28,45). In this mechanism of translation initi...

Upregulated c-myc expression in multiple myeloma by internal ribosome entry results from increased interactions with and expression of PTB-1 and YB-1

“…It has been shown that the 5 0 untranslated region of c-myc, which is encoded by exon 1, contains an IRES (Nanbru et al, 1997;Stoneley et al, 1998) that functions to maintain c-myc expression during a number of pathophysiological conditions including apoptosis , genotoxic stress (Subkhankulova et al, 2001) and viral infection (Johannes and Sarnow, 1998). c-myc IRESmediated translation requires both canonical initiation factors (Spriggs et al, 2009) and specific IRES-transacting factors (ITAFs) and these have been shown to include Y-box binding protein 1 (YB-1), PSF, P54nrb, GRSF1, polypyrimidine tract-binding protein 1 (PTB-1), hnRNPK and hnRNPA1 (Evans et al, 2003;Cobbold et al, 2008;Jo et al, 2008). It has been proposed that ITAFs stimulate IRES-mediated translation by acting as RNA chaperones and allowing the IRES to adopt the correct conformation so that the 40S ribosomal subunit can bind (Stoneley and Willis, 2004b).…”

Structural basis of G-tract recognition and encaging by hnRNP F quasi-RRMs

“…Most intermolecular nOes were derived from filtered-edited NOESY experiments using 15 N- 13 C labeled proteins and unlabeled RNA ( Supplementary Fig. 2a).…”

“…We would like to thank D. Black for providing the clone of full-length hnRNP F and S. Pitsch for providing sugar- 13 …”

“…Some of the trans-acting factors that bind to these G-tracts belong to the heterogeneous nuclear ribonucleoprotein (hnRNP) F/H family of proteins that consists of five members (hnRNP F, hnRNP H, hnRNP H', hnRNP 2H9, and GRich Sequence Factor (GRSF) 1) 9,10 . GRSF-1 is mainly involved in translation regulation 11,12 and Internal Ribosome Entry Site (IRES) mediated translation 13 , while hnRNP H, H' and F are mostly involved in the regulation of alternative splicing [14][15][16][17][18][19][20][21][22][23] and polyadenylation [24][25][26] . The posttranscriptional regulation by hnRNP F/H proteins is a crucial event since mutations in G-tracts often result in aberrant splicing patterns and can be associated with various diseases [27][28][29][30][31][32] .…”

La Autoantigen Mediates Oxidant Induced De Novo Nrf2 Protein Translation

“…The IRES of XIAP has been linked to binding of La autoantigen, hnRNPC1/2 and DAP5 (7). A recent study using proteomic approaches found four proteins: G-rich RNA sequence binding factor 1 (GRSF-1), Y-box binding protein 1 (YB-1), PTB associated splicing factor (PSF), and its binding partner p54nrb, as ITAFs for regulating translation of myc family genes (10). In this study, we report the finding of La autoantigen as an ITAF regulating Nrf2 protein translation under oxidative stress.…”