Eukaryotic translation initiation factor 4GI and p97 promote cellular internal ribosome entry sequence-driven translation.
Numerous cellular mRNAs encoding proteins critical during cell stress, apoptosis, and the cell cycle seem to be translated by means of internal ribosome entry sequences (IRES) when cap-dependent translation is compromised. The underlying molecular mechanisms are largely unknown. Using a HeLa-based cell-free translation system that mirrors the function of cellular IRESs in vitro, we recently demonstrated that translation from the c-myc IRES continues after proteolytic cleavage of eukaryotic translation initiation factor (eIF) 4G. To address the role of eIF4G in cellular IRES-driven translation directly, we immunodepleted eIF4GI from the HeLa cell translation extracts. After efficient depletion of eIF4GI (>90%), both cap-dependent and c-myc IRES-dependent translations are diminished to residual levels (<5%). In striking contrast to cap-dependent translation, c-myc IRES-dependent translation is fully restored by addition of the conserved middle fragment of eIF4GI, harboring the eIF3- and eIF4A-binding sites. p97, an eIF4G-related protein that has been described both as an inhibitor of translation and as a modulator of apoptosis, not only suffices to also rescue c-myc IRES-driven (but not cap-dependent) translation, but it even superinduces IRES-mediated translation 3-fold compared with nondepleted extracts. Interestingly, both p97 and the middle fragment of eIF4GI also rescue translation driven by proapoptotic (p97) and antiapoptotic [X-linked inhibitor of apoptosis (XIAP) and cellular inhibitor of apoptosis 1 (c-IAP1)] IRESs, reflecting a broader role of these polypeptides in cellular IRES-mediated translation and indicating their importance in apoptosis.
Hundsdoerfer
Patrick
P
Gene Expression Unit, European Molecular Biology Laboratory, Meyerhofstrasse 1, D-69117 Heidelberg, Germany.
Thoma
Christian
C
Hentze
Matthias W
MW
D020121:5' Untranslated Regions / Q000235:genetics / Q000378:metabolism*; D017209:Apoptosis / Q000502:physiology; D002453:Cell Cycle / Q000502:physiology; D002474:Cell-Free System / Q000378:metabolism; D039562:Eukaryotic Initiation Factor-4F / Q000378:metabolism*; D006367:HeLa Cells; D006801:Humans; D051034:Inhibitor of Apoptosis Proteins; D014176:Protein Biosynthesis / Q000502:physiology*; D011506:Proteins / Q000378:metabolism; D016271:Proto-Oncogene Proteins c-myc / Q000235:genetics / Q000378:metabolism*; D012315:RNA Caps / Q000235:genetics / Q000378:metabolism; D038621:Regulatory Sequences, Ribonucleic Acid / Q000502:physiology*; D044767:Ubiquitin-Protein Ligases; D051636:X-Linked Inhibitor of Apoptosis Protein
D020121:5' Untranslated Regions; D039562:Eukaryotic Initiation Factor-4F; D051034:Inhibitor of Apoptosis Proteins; D011506:Proteins; D016271:Proto-Oncogene Proteins c-myc; D012315:RNA Caps; D038621:Regulatory Sequences, Ribonucleic Acid; D051636:X-Linked Inhibitor of Apoptosis Protein; C496866:XIAP protein, human; C494833:BIRC2 protein, human; D044767:Ubiquitin-Protein Ligases