Role of a ribosome-associated E3 ubiquitin ligase in protein quality control

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Abstract
Translation of mRNAs lacking stop codons (nonstop mRNA) results in the production of aberrant proteins that may have deleterious effects on cellular function. It is unclear how eukaryotic cells eliminate such 'nonstop' proteins. In this study, Mario Bengtson and Claudio Joazeiro demonstrate that in the yeast Saccharomyces cerevisiae, an E3 ubiquitin ligase called Ltn1 acts in the quality control pathway. It associates with ribosomes and marks nonstop proteins that are then targeted for proteasomal degradation by the ubiquitin pathway. Ltn1-mediated ubiquitylation is triggered when the proteins stall at the poly(A) tail in the ribosomes on translation. Loss of Ltn1 function conferred increased cellular stress as a result of elevated levels of nonstop proteins. The translation of messenger RNA that lacks stop codons results in the production of aberrant proteins, which may have harmful effects on the cell. It is unclear how eukaryotic cells eliminate these 'non-stop' proteins. Here it is shown that, in Saccharomyces cerevisiae, an E3 ubiquitin ligase called Ltn1 acts in the quality-control pathway. It associates with ribosomes and marks non-stop proteins with ubiquitin, which targets the proteins for degradation. Messenger RNA lacking stop codons (‘non-stop mRNA’) can arise from errors in gene expression, and encode aberrant proteins whose accumulation could be deleterious to cellular function1,2. In bacteria, these ‘non-stop proteins’ become co-translationally tagged with a peptide encoded by ssrA/tmRNA (transfer-messenger RNA), which signals their degradation by energy-dependent proteases1,3. How eukaryotic cells eliminate non-stop proteins has remained unknown. Here we show that the Saccharomyces cerevisiae Ltn1 RING-domain-type E3 ubiquitin ligase acts in the quality control of non-stop proteins, in a process that is mechanistically distinct but conceptually analogous to that performed by ssrA: Ltn1 is predominantly associated with ribosomes, and it marks nascent non-stop proteins with ubiquitin to signal their proteasomal degradation. Ltn1-mediated ubiquitylation of non-stop proteins seems to be triggered by their stalling in ribosomes on translation through the poly(A) tail. The biological relevance of this process is underscored by the finding that loss of Ltn1 function confers sensitivity to stress caused by increased non-stop protein production. We speculate that defective protein quality control may underlie the neurodegenerative phenotype that results from mutation of the mouse Ltn1 homologue Listerin.