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


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.

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Figure 1: The yeast Listerin/Ltn1 E3 ligase functions in quality control of non-stop proteins.
Figure 2: Ltn1 targets newly synthesized non-stop proteins.
Figure 3: Nascent polylysine peptides stall in ribosomes, cause translational arrest and trigger Ltn1-mediated ubiquitylation.
Figure 4: Ltn1 is predominantly associated with ribosomes.
Figure 5: Ltn1 confers resistance to stress caused by non-stop protein production.


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We thank M. C. Sogayar for continued mentorship; I. Kwan for technical help; A. Bacconi for help with microscopy; T. Inada, A. van Hoof, S. Liebman and J. Frydman for reagents; the Williamson and Saez laboratories for help with sucrose-gradient fractionation and real-time PCR, respectively; E. Peters for mass spectrometry analyses; M. Smolka for help with yeast methods; and R. Deshaies, E. P. Geiduschek and T. Hunter for discussions. Work in the Joazeiro laboratory is supported by Research Scholar Grant 08-298-01-TBE from the American Cancer Society and grant R01GM083060 from the National Institute of General Medical Sciences.

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C.A.P.J. and M.H.B. designed the studies, interpreted the data and wrote the manuscript. M.H.B. conducted the experiments.

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Correspondence to Claudio A. P. Joazeiro.

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The authors declare no competing financial interests.

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This file contains Supplementary Methods, a List of Genotypes, Supplementary Figures 1-9 with legends, Supplementary Table 1, and additional references. (PDF 3181 kb)

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Bengtson, M., Joazeiro, C. Role of a ribosome-associated E3 ubiquitin ligase in protein quality control. Nature 467, 470–473 (2010).

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