Eukaryotic Initiation Factor 5B (eIF5B) Cooperates with eIF1A and eIF5 to Facilitate uORF2-Mediated Repression of ATF4 Translation

• Main Authors: Joseph A. Ross, Kamiko R. Bressler, Nehal Thakor
• Format: Article
• Language: English
• Published: MDPI AG 2018-12-01
• Series: International Journal of Molecular Sciences
• Subjects: eukaryotic initiation factor 5B (eIF5B); eIF1A; eIF2A; upstream open reading frames (uORFs); Activating Transcription Factor 4 (ATF4); eukaryotic initiation factor 2α (eIF2α)
• Online Access: https://www.mdpi.com/1422-0067/19/12/4032

A variety of cellular stresses lead to global translation attenuation due to phosphorylation of the alpha subunit of eukaryotic initiation factor 2 (eIF2), which decreases the availability of the eIF2-GTP-Met-tRNA_i ternary complex. However, a subset of mRNAs continues to be translated by non-canonical mechanisms under these conditions. In fact, although translation initiation of activating transcription factor 4 (ATF4) is normally repressed by an upstream open reading frame (uORF), a decreased availability of ternary complex leads to increased translation of the main ATF4-coding ORF. We show here that siRNA-mediated depletion of eIF5B&#8212;which can substitute for eIF2 in delivering Met-tRNA_i&#8212;leads to increased levels of ATF4 protein in mammalian cells. This de-repression is not due to phosphorylation of eIF2&#945; under conditions of eIF5B depletion. Although eIF5B depletion leads to a modest increase in the steady-state levels of <i>ATF4</i> mRNA, we show by polysome profiling that the depletion of eIF5B enhances ATF4 expression primarily at the level of translation. Moreover, eIF5B silencing increases the expression of an ATF4-luciferase translational reporter by a mechanism requiring the repressive uORF2. Further experiments suggest that eIF5B cooperates with eIF1A and eIF5, but not eIF2A, to facilitate the uORF2-mediated repression of ATF4 translation.