DAP5 and IRES-mediated translation during programmed cell death

Abstract

DAP5 (Death Associated Protein 5), also named p97 and NAT1, is a member of the eIF4G family that lacks the eIF4E binding site. Its function was linked to programmed cell death (PCD) based on the seminal finding that a fragment of DAP5/ p97 protein, which acted in a dominant-negative manner, protected against IFN-gamma (IFN-g)-induced cell death. Subsequently, it was found that DAP5 protein is activated during cell death by caspase cleavage, yielding a C-terminaltruncated protein of 86 kDa. The p86 form promotes internal ribosome entry site (IRES)-dependent translation of several mRNAs including Apaf-1, c-myc, XIAP, HIAP2 and DAP5 itself, which carries an IRES element in its 5 0 UTR. The activation of DAP5 IRES creates a positive feedback loop, which results in sustained translation of DAP5 protein during PCD under conditions of reduced cap-dependent translation. DAP5 deficiency prevents embryonic stem (ES) cell differentiation, suggesting a wider range of DAP5 mRNA targets and additional mechanisms that might activate the protein.

Searching for IRES

“…Our interest in this case is in IRESspecific cellular trans-acting factors (ITAFs). Some of the mRNA binding proteins implicated in IRES-mediated translation are polypyrimidine tract binding protein PTB/ hnRNP I (Giraud et al 2001;Mitchell et al 2001Mitchell et al , 2003Mitchell et al , 2005Pickering et al 2003;Cho et al 2005); La autoantigen Bhattacharyya and Das 2005;Marash and Kimchi 2005); hnRNP A1 (Bonnal et al 2005); hnRNP C1/C2 (Sella et al 1999;Millard et al 2000;Holcik et al 2003); hnRNP E, hnRNP K, DAP5/ p86 (Henis-Korenblit et al 2000;Nevins et al 2003;Warnakulasuriyarachchi et al 2004;Marash and Kimchi 2005); Unr (Mitchell et al 2001Tinton et al 2005); p60 (Vagner et al 1996), HuR (Millard et al 2000), and PCBP1 (Pickering et al 2003). For a more comprehensive list, see the online IRES database (http://ifr31w3.toulouse.…”

“…When eIF4E, the cap binding protein, has been removed (Hernandez et al 2004), the Drosophila reaper IRES initiates translation more efficiently than capped messages. The eIF4G family member DAP5/p97 is cleaved to DAP5/p86 and enhances IRES-mediated translation (Henis-Korenblit et al 2002;Nevins et al 2003;Warnakulasuriyarachchi et al 2004;Marash and Kimchi 2005). The importance of ribosomal proteins in IRES translation initiation was shown using a genome-wide RNAi screen of Drosophila genes.…”

The eIF4G–homolog p97 can activate translation independent of caspase cleavage

“…It further interacts through its C terminus with the eIF4E-kinase Mnk1, another property it shares with eIF4G (Pyronnet et al 1999). Importantly, p97 lacks regions found at the N terminus of eIF4G, which interact with eIF4E and PABP Marash and Kimchi 2005). Thus, p97 is thought to be incapable of supporting canonical cap-dependent mRNA translation.…”

Direct ribosomal binding by a cellular inhibitor of translation

“…(b) Phosphorylation of eIF2α in the presence or absence of Reaper. Translating RRL not supplemented with hemin, or supplemented with hemin and either a control peptide (Reaper [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] , final concentration of 325 µM) or active Reaper peptide (Reaper or full-length Reaper, final concentration of 40 µM) was resolved by isoelectric focusing and immunoblotted with antibodies directed against eIF2α to detect eIF2α phosphorylation. Not supplementing RRL with hemin results in activation of HRI kinase, which phosphorylates eIF2α and inhibits translation.…”

“…The 40S subunit is thought to scan linearly along the mRNA until it engages the first AUG codon located within the proper context 11 . The interaction between the 40S subunit and the mRNA and the subsequent translation initiation steps are chaperoned by eIF regulatory proteins 1,2,[5][6][7] . In canonical capdependent translation, initiation factors have many roles: they prevent premature subunit association, recruit the initiator transfer RNA to the 40S subunit to form a 43S complex, recruit the 43S complex to the mRNA, unwind the mRNA during ribosomal scanning and facilitate correct start-codon recognition [12][13][14][15][16] .…”