ZNF598 and the Ribosome-associated Quality Trigger (RQT) complex dissociate a ribosome stalled on a no-go mRNA

ZNF598 and the Ribosome-associated Quality Trigger (RQT) complex dissociate a ribosome stalled on a no-go mRNA Due to features such as damaged nucleotides, strong secondary structure, presence of stretches of more than four uninterrupted AAA codons in a mRNA coding region (designated no-go mRNAs) a ribosome translating an mRNA can stall and cause collisions with the ribosomes trailing it on the mRNA (reviewed in Joazeiro 2017, Eisenack and Trentini 2022, Filbeck et al. 2022, McGirr et al. 2025). In cases in which the ribosome stalls internally on the mRNA and a 3' region of the mRNA protrudes from the ribosome, ZNF598, a ubiquitin E3 ligase that is the homolog of yeast HEL2, binds the ribosome (Garzia et al. 2017, Juszkiewicz and Hegde 2017, Sundaramoorthy et al. 2017) and ubiquitinates the 40S subunit ribosomal proteins eS10 (RPS10) at residues K138 and K139 and uS10 (RPS20) at residues K4 and K8 to initiate splitting of the 40S and 60S ribosomal subunits (Juszkiewicz and Hegde 2017, Sundaramoorthy et al. 2017, Garzia et al. 2017, Juszkiewicz et al. 2018, Narita et al. 2022, Miścicka et al. 2024, also inferred from yeast homologs in Matsuo et al. 2017, Sitron et al. 2017, Ikeuchi et al. 2019, reviewed in Ford et al. 2024). ZAKα also interacts with the collided ribosome at the 18S helix through its c-terminal domain, which triggers ZAKα autophosphorylation and phosphorylation of p38/JNK in a MAPK cascade known as the ribotoxic stress response (Wu et al. 2020, Vind et al. 2020, Stoneley et al. 2022).
The ASCC2 subunit (Narita et al. 2022) of the ribosome quality control trigger complex (RQT complex, ASCC2:ASCC3:TRIP4) binds K63-linked polyubiquitin conjugated to the 40S protein uS10 (Hashimoto et al. 2020, Juszkiewicz et al. 2020, Narita et al. 2022, and inferred from yeast homologs in Matsuo et al. 2023). The ASCC3 subunit of the RQT complex splits stalled 80S ribosomes with K63-polyubiquitinated uS10 into 60S and 40S subunits (Hashimoto et al. 2020, Juszkiewicz et al. 2020, Narita et al. 2022, Miścicka et al. 2024) apparently by exerting a pulling force on the mRNA (inferred from the yeast homolog Slh1 in Best et al. 2023). The peptidyl-tRNA remains bound in the 60S subunit, with the tRNA positioned in the P site. The problematic mRNA dissociates after splitting and is thought to be degraded at this time. In the case of collided yeast ribosomes, the mRNA is first endonucleolytically cleaved and the cleavage products are exonucleolytically degraded by XRN1 and the exosome (Ikeuchi et al. 2019).