Phosphorylation of a reinitiation supporting protein, RISP, determines its function in translation reinitiation.

Reinitiation supporting protein, RISP, interacts with 60S (60S ribosomal subunit) and eIF3 (eukaryotic initiation factor 3) in plants. TOR (target-of-rapamycin) mediates RISP phosphorylation at residue Ser267, favoring its binding to eL24 (60S ribosomal protein L24). In a viral context, RISP, when phosphorylated, binds the CaMV transactivator/ viroplasmin, TAV, to assist in an exceptional mechanism of reinitiation after long ORF translation. Moreover, we show here that RISP interacts with eIF2 via eIF2ß and TOR downstream target 40S ribosomal protein eS6. A RISP phosphorylation knockout, RISP-S267A, binds preferentially eIF2ß, and both form a ternary complex with eIF3a in vitro. Accordingly, transient overexpression in plant protoplasts of RISP-S267A, but not a RISP phosphorylation mimic, RISP-S267D, favors translation initiation. In contrast, RISP-S267D preferentially binds eS6, and, when bound to the C-terminus of eS6, can capture 60S in a highly specific manner in vitro, suggesting that it mediates 60S loading during reinitiation. Indeed, eS6-deficient plants are highly resistant to CaMV due to their reduced reinitiation capacity. Strikingly, an eS6 phosphomimic, when stably expressed in eS6-deficient plants, can fully restore the reinitiation deficiency of these plants in cellular and viral contexts. These results suggest that RISP function in translation (re)initiation is regulated by phosphorylation at Ser267.

Structural Insights into the Role of Diphthamide on Elongation Factor 2 in mRNA Reading-Frame Maintenance.

One of the most critical steps of protein biosynthesis is the coupled movement of mRNA, which encodes genetic information, with tRNAs on the ribosome. In eukaryotes, this process is catalyzed by a conserved G-protein, the elongation factor 2 (eEF2), which carries a unique post-translational modification, called diphthamide, found in all eukaryotic species. Here we present near-atomic resolution cryo-electron microscopy structures of yeast 80S ribosome complexes containing mRNA, tRNA and eEF2 trapped in different GTP-hydrolysis states which provide further structural insights into the role of diphthamide in the mechanism of translation fidelity in eukaryotes.

The cryo-EM Structure of a Novel 40S Kinetoplastid-Specific Ribosomal Protein.

Kinetoplastids are potentially lethal protozoan pathogens affecting more than 20 million people worldwide. There is a critical need for more specific targets for the development of safer anti-kinetoplastid therapeutic molecules that can replace the scarce and highly cytotoxic current drugs. The kinetoplastid ribosome represents a potential therapeutic target due to its relative structural divergence when compared with its human counterpart. However, several kinetoplastid-specific ribosomal features remain uncharacterized. Here, we present the near-atomic cryoelectron microscopy structure of a novel bona fide kinetoplastid-specific ribosomal (r-) protein (KSRP) bound to the ribosome. KSRP is an essential protein located at the solvent face of the 40S subunit, where it binds and stabilizes kinetoplastid-specific domains of rRNA, suggesting its role in ribosome integrity. KSRP also interacts with the r-protein eS6 at a region that is only conserved in kinetoplastids. The kinetoplastid-specific ribosomal environment of KSRP provides a promising target for the design of safer anti-kinetoplastidian drugs.

ABCE1: A special factor that orchestrates translation at the crossroad between recycling and initiation.

For many years initiation and termination of mRNA translation have been studied separately. However, a direct link between these 2 isolated stages has been suggested by the fact that some initiation factors also control termination and can even promote ribosome recycling; i.e. the last stage where post-terminating 80S ribosomes are split to start a new round of initiation. Notably, it is now firmly established that, among other factors, ribosomal recycling critically requires the NTPase ABCE1. However, several earlier reports have proposed that ABCE1 also somehow participates in the initiation complex assembly. Based on an extended analysis of our recently published late-stage 48S initiation complex from rabbit, here we provide new mechanistic insights into this putative role of ABCE1 in initiation. This point of view represents the first structural evidence in which the regulatory role of the recycling factor ABCE1 in initiation is discussed and establishes a corner stone for elucidating the interplay between ABCE1 and several initiation factors during the transit from ribosomal recycling to formation of the elongation competent 80S initiation complex.

The Arabidopsis TOR Kinase Specifically Regulates the Expression of Nuclear Genes Coding for Plastidic Ribosomal Proteins and the Phosphorylation of the Cytosolic Ribosomal Protein S6.

Protein translation is an energy consuming process that has to be fine-tuned at both the cell and organism levels to match the availability of resources. The target of rapamycin kinase (TOR) is a key regulator of a large range of biological processes in response to environmental cues. In this study, we have investigated the effects of TOR inactivation on the expression and regulation of Arabidopsis ribosomal proteins at different levels of analysis, namely from transcriptomic to phosphoproteomic. TOR inactivation resulted in a coordinated down-regulation of the transcription and translation of nuclear-encoded mRNAs coding for plastidic ribosomal proteins, which could explain the chlorotic phenotype of the TOR silenced plants. We have identified in the 5' untranslated regions (UTRs) of this set of genes a conserved sequence related to the 5' terminal oligopyrimidine motif, which is known to confer translational regulation by the TOR kinase in other eukaryotes. Furthermore, the phosphoproteomic analysis of the ribosomal fraction following TOR inactivation revealed a lower phosphorylation of the conserved Ser240 residue in the C-terminal region of the 40S ribosomal protein S6 (RPS6). These results were confirmed by Western blot analysis using an antibody that specifically recognizes phosphorylated Ser240 in RPS6. Finally, this antibody was used to follow TOR activity in plants. Our results thus uncover a multi-level regulation of plant ribosomal genes and proteins by the TOR kinase.

eIF3 Peripheral Subunits Rearrangement after mRNA Binding and Start-Codon Recognition.

mRNA translation initiation in eukaryotes requires the cooperation of a dozen eukaryotic initiation factors (eIFs) forming several complexes, which leads to mRNA attachment to the small ribosomal 40S subunit, mRNA scanning for start codon, and accommodation of initiator tRNA at the 40S P site. eIF3, composed of 13 subunits, 8 core (a, c, e, f, h, l, k, and m) and 5 peripheral (b, d, g, i, and j), plays a central role during this process. Here we report a cryo-electron microscopy structure of a mammalian 48S initiation complex at 5.8 Å resolution. It shows the relocation of subunits eIF3i and eIF3g to the 40S intersubunit face on the GTPase binding site, at a late stage in initiation. On the basis of a previous study, we demonstrate the relocation of eIF3b to the 40S intersubunit face, binding below the eIF2-Met-tRNAi(Met) ternary complex upon mRNA attachment. Our analysis reveals the deep rearrangement of eIF3 and unravels the molecular mechanism underlying eIF3 function in mRNA scanning and timing of ribosomal subunit joining.

TOR and S6K1 promote translation reinitiation of uORF-containing mRNAs via phosphorylation of eIF3h.

Mammalian target-of-rapamycin (mTOR) triggers S6 kinase (S6K) activation to phosphorylate targets linked to translation in response to energy, nutrients, and hormones. Pathways of TOR activation in plants remain unknown. Here, we uncover the role of the phytohormone auxin in TOR signalling activation and reinitiation after upstream open reading frame (uORF) translation, which in plants is dependent on translation initiation factor eIF3h. We show that auxin triggers TOR activation followed by S6K1 phosphorylation at T449 and efficient loading of uORF-mRNAs onto polysomes in a manner sensitive to the TOR inhibitor Torin-1. Torin-1 mediates recruitment of inactive S6K1 to polysomes, while auxin triggers S6K1 dissociation and recruitment of activated TOR instead. A putative target of TOR/S6K1-eIF3h-is phosphorylated and detected in polysomes in response to auxin. In TOR-deficient plants, polysomes were prebound by inactive S6K1, and loading of uORF-mRNAs and eIF3h was impaired. Transient expression of eIF3h-S178D in plant protoplasts specifically upregulates uORF-mRNA translation. We propose that TOR functions in polysomes to maintain the active S6K1 (and thus eIF3h) phosphorylation status that is critical for translation reinitiation.

Expression profile of maize (Zea mays L.) embryonic axes during germination: translational regulation of ribosomal protein mRNAs.

Seed germination is a critical developmental period for plant propagation. Information regarding gene expression within this important period is relevant for understanding the main biochemical processes required for successful germination, particularly in maize, one of the most important cereals in the world. The present research focuses on the global microarray analysis of differential gene expression between quiescent and germinated maize embryo stages. This analysis revealed that a large number of mRNAs stored in the quiescent embryonic axes (QEAs) were differentially regulated during germination in the 24 h germinated embryonic axes (GEAs). These genes belong to 14 different functional categories and most of them correspond to metabolic processes, followed by transport, transcription and translation. Interestingly, the expression of mRNAs encoding ribosomal proteins [(r)-proteins], required for new ribosome formation during this fast-growing period, remains mostly unchanged throughout the germination process, suggesting that these genes are not regulated at the transcriptional level during this developmental period. To investigate this issue further, comparative microarray analyses on polysomal mRNAs from growth-stimulated and non-stimulated GEAs were performed. The results revealed that (r)-protein mRNAs accumulate to high levels in polysomes of the growth-stimulated tissues, indicating a translational control mechanism to account for the rapid (r)-protein synthesis observed within this period. Bioinformatic analysis of (r)-protein mRNAs showed that 5' TOP (tract of pyrimidines)-like sequences are present only in the 5'-untranslated region set of up-regulated (r)-protein mRNAs. This overall approach to the germination process allows an in-depth view of molecular changes, enabling a broader understanding of the regulatory mechanisms that occur during this process.