Abstract
The cellular levels and activities of ribosomes directly regulate gene expression during numerous physiological processes. The mechanisms that globally repress translation are incompletely understood. Here, we use electron cryomicroscopy to analyze inactive ribosomes isolated from mammalian reticulocytes, the penultimate stage of red blood cell differentiation. We identify two types of ribosomes that are translationally repressed by protein interactions. The first comprises ribosomes sequestered with elongation factor 2 (eEF2) by SERPINE mRNA binding protein 1 (SERBP1) occupying the ribosomal mRNA entrance channel. The second type are translationally repressed by a novel ribosome-binding protein, interferon-related developmental regulator 2 (IFRD2), which spans the P and E sites and inserts a C-terminal helix into the mRNA exit channel to preclude translation. IFRD2 binds ribosomes with a tRNA occupying a noncanonical binding site, the 'Z site', on the ribosome. These structures provide functional insights into how ribosomal interactions may suppress translation to regulate gene expression.
Highlights
Translation is an important point of regulation for gene expression
In addition to observing ribosomes silenced by SERPINE mRNA binding protein 1 (SERBP1), we identify interferon-related developmental regulator 2 (IFRD2) as a novel factor capable of translationally inactivating ribosomes
Identification of translationally inactive ribosomes in reticulocyte lysate We previously used a cell-free translation system derived from rabbit reticulocyte lysate to isolate various ribosomal complexes for cryo-EM analysis (Brown et al, 2015b; Shao et al, 2016)
Summary
Translation is an important point of regulation for gene expression. The overall levels and activities of ribosomes are implicated in cellular differentiation, developmental disorders, and cancers (Buszczak et al, 2014; Narla and Ebert, 2010). Translation is optimized for hemoglobin production during red blood cell differentiation (Mills et al, 2016; Smith and McNamara, 1971). This translational program occurs predominantly in enucleated cells, precluding transcriptional control and relying solely on preexisting ribosomes and translational factors before the ribosomes are degraded upon terminal differentiation (Rowley, 1965). We observe tRNA in this site in the presence and absence of either IFRD2 and P-site tRNA Together, these findings identify new ribosomal interactions that may modulate global translation activity during erythropoiesis and in other differentiating cells
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