Project description:Impairment of ribosome function activates the MAPKKK ZAK, leading to activation of mitogen-activated protein (MAP) kinases p38 and JNK and inflammatory signaling. The mechanistic basis for activation of this ribotoxic stress response (RSR) re-mains completely obscure. We show that the long isoform of ZAK (ZAKα) directly associates with ribosomes by inserting its flexible C terminus into the ribosomal intersubunit space. Here, ZAKα binds helix14 of 18S ribosomal RNA (rRNA). An adjacent domainin ZAKa also probes the ribosome, and together, these sensor domains are critically required for RSR activation after inhibition of both the E-site, the peptidyl transferase center (PTC), and ribotoxinaction. Finally, we show that ablation of the RSR response leads to organismal phenotypes and decreased lifespan in the nematode Caenorhabditis elegans (C. elegans). Our findings yield mechanistic insight into how cells detect ribotoxic stress and provide experimental in vivo evidence for its physiological importance.
Project description:Ribosome stalling at problematic sequences in mRNAs leads to collisions that trigger a collection of quality control events including ribosome rescue, targeting the nascent polypeptide for decay (Ribosome-mediated Quality Control or RQC), and targeting of the mRNA for decay (No Go Decay or NGD). Using a reverse genetic screen in yeast, we identify Cue2 as the endonuclease that is recruited to stalled ribosomes to promote NGD. Following Cue2-mediated cleavage, ribosomes upstream of the cleavage site translate to the end of the truncated mRNA and are rescued by the Dom34:Hbs1 complex. We also show that the putative helicase Slh1 (part of the RQC Trigger or RQT complex) removes collided ribosomes behind the lead stalled ribosome and thereby reduces endonucleolytic cleavage by Cue2. The synergistic activities of Cue2 and Slh1 define two parallel pathways that allow cells to recognize and respond to ribosomes trapped on problematic mRNAs.
Project description:Cells can respond to stalled ribosomes by sensing ribosome collisions and employing quality control pathways. How ribosome stalling is resolved without collisions, however, has remained elusive. Here, focusing on non-colliding stalling exhibited by decoding-defective ribosomes, we identified Fap1 as a stalling sensor triggering 18S non-functional rRNA decay via poly-ubiquitination of uS3. Ribosome profiling revealed an enrichment of Fap1 at the translation initiation site but also association with elongating individual ribosomes. Cryo-EM structures of Fap1-bound ribosomes elucidated Fap1 probing the mRNA simultaneously at both the entry and exit channels suggesting a mRNA stasis sensing activity, and Fap1 sterically hinders formation of canonical collided di-ribosomes. Our findings indicate that individual stalled ribosomes are the potential signal for ribosome dysfunction, leading to accelerated turnover of the ribosome itself.
Project description:Impairment of translation can lead to stalling and collision of ribosomes which constitute an activation platform for several ribosomal stress-surveillance pathways. Among these is the Ribotoxic Stress Response (RSR), where ribosomal sensing by the MAP3K ZAKa leads to activation of p38 and JNK kinases. Despite these insights, the physiological ramifications of ribosomal impairment and downstream RSR signaling remain elusive. Here we show that stalling of ribosomes is sufficient to activate ZAKa. In response to amino acid deprivation and full nutrient starvation, RSR impacts on the ensuing metabolic responses in cells, nematodes and mice. The RSR-regulated responses in these model systems include regulation of AMPK and mTOR signaling, survival under starvation conditions, stress hormone production and regulation of blood sugar control. In addition, ZAK-/- mice present with a lean phenotype. Our work highlights stalled ribosomes as metabolic signals and demonstrates a role for RSR signaling in metabolic regulation.
Project description:mass spec raw files for the manuscript from Oltion, K. et. al "An E3 ligase network engages GCN1 to promote degradation of translation factors on stalled ribosomes"
Project description:Translation elongation stalling has the potential to produce toxic truncated protein fragments. Translation of either non-stop mRNA or transcripts coding for poly-basic sequences induces ribosome stalling, and the arrest product is degraded by the ribosome-mediated quality control (RQC) system. During this process, the stalled ribosome is dissociated into subunits, and the polypeptide is ubiquitinated by the E3 ubiquitin ligase Listerin on the 60S large ribosomal subunit, leading to subsequent proteasomal degradation. However, it is largely unknown how the specific stalled ribosomes are recognized as aberrant to engage the RQC system. Here, we report that ubiquitination of the ribosomal protein uS10 of the 40S small ribosomal subunit, by the E3 ubiquitin ligase Hel2 (or RQC-trigger (Rqt) 1) initiates RQC. We identified a novel RQC-trigger (RQT) complex composed of the RNA helicase-family protein Slh1/Rqt2, the ubiquitin binding protein Cue3/Rqt3, and yKR023W/Rqt4 that is required for RQC. The defects in RQC of the RQT mutants correlated with sensitivity to anisomycin, which stalls ribosome at the rotated form, suggesting that RQT factors rescue ribosomes stalled by this drug. Our un-biased survey by ribosome profiling revealed that ribosomes stalled at the rotated state with specific pairs of codons at P-A sites serve as RQC substrates. Rqt1 specifically ubiquitinates these arrested ribosomes to target them to the RQT complex, allowing subsequent RQC reactions including dissociation of the stalled ribosome into subunits. Our results provide mechanistic insight into the surveillance system for aberrant proteins induced by ribosome stalling and mediated by ribosome ubiquitination.
Project description:Translation of damaged mRNA can lead to ribosome stalling, thereby producing incomplete proteins toxic to the cell. The mechanism of ribosome-associated quality control (RQC) disassembles stalled ribosomes through the actions of the ASC-1 complex (ASCC). Here, we show that some reagents that chemically damage RNA, such as ultraviolet light (UV), cause ribosome stalling, which leads to accumulation of the ASC-1 complex (ASCC) on stalled ribosomes and stable interaction of the ASCC3 helicase with RNA. In contrast, the ASCC was not similarly affected by emetine or anisomycin-induced ribosome stalling. Our work identified two different types of stalled ribosome. Ribosomes arrested by emetine or anisomycin are transient as they are resolved by the ASCC. Whereas the ASCC fails to split some stalled ribosomes, such as those induced by UV, resulting in long-lived stalled ribosome complexes. We show that ribosome stalling activates the G1/S and G2/M cell cycle checkpoints with long-lived stalled ribosomes causing prolonged checkpoint activation. Thus, the cell adjusts this adaptive survival response to match the nature of the stalled ribosome.
Project description:Translation of damaged mRNA can lead to ribosome stalling, thereby producing incomplete proteins toxic to the cell. The mechanism of ribosome-associated quality control (RQC) disassembles stalled ribosomes through the actions of the ASC-1 complex (ASCC). Here, we show that some reagents that chemically damage RNA, such as ultraviolet light (UV), cause ribosome stalling, which leads to accumulation of the ASC-1 complex (ASCC) on stalled ribosomes and stable interaction of the ASCC3 helicase with RNA. In contrast, the ASCC was not similarly affected by emetine or anisomycin-induced ribosome stalling. Our work identified two different types of stalled ribosome. Ribosomes arrested by emetine or anisomycin are transient as they are resolved by the ASCC. Whereas the ASCC fails to split some stalled ribosomes, such as those induced by UV, resulting in long-lived stalled ribosome complexes. We show that ribosome stalling activates the G1/S and G2/M cell cycle checkpoints with long-lived stalled ribosomes causing prolonged checkpoint activation. Thus, the cell adjusts this adaptive survival response to match the nature of the stalled ribosome.