Ribosome stalling on eukaryotic mRNAs triggers cotranslational RNA and protein degradation through conserved mechanisms. For example, mRNAs lacking a stop codon are degraded by the exosome in association with its cofactor, the SKI complex, whereas the corresponding aberrant nascent polypeptides are ubiquitinated by the E3 ligases Ltn1 and Not4 and become proteasome substrates. How translation arrest is linked with polypeptide degradation is still unclear. Genetic screens with SKI and LTN1 mutants allowed us to identify translation-associated element 2 (Tae2) and ribosome quality control 1 (Rqc1), two factors that we found associated, together with Ltn1 and the AAA-ATPase Cdc48, to 60S ribosomal subunits. Translation-associated element 2 (Tae2), Rqc1, and Cdc48 were all required for degradation of polypeptides synthesized from NonStop mRNAs (Non-Stop protein decay; NSPD). Both Ltn1 and Rqc1 were essential for the recruitment of Cdc48 to 60S particles. Polysome gradient analyses of mutant strains revealed unique intermediates of this pathway, showing that the polyubiquitination of Non-Stop peptides is a progressive process. We propose that ubiquitination of the nascent peptide starts on the 80S and continues on the 60S, on which Cdc48 is recruited to escort the substrate for proteasomal degradation.quality control | Saccharomyces cerevisiae

Prion proteins are found in mammals and yeast, and can transmit diseases and encode heritable phenotypic traits. In Saccharomyces cerevisiae, eRF3, Rnq1, Ure2 and Swil are functional proteins with a soluble conformation that can switch to a non-functional, amyloid conformation denoted as [PSI+], [PIN+], [URE3] and [SWI+], respectively. The prion [PSI+] corresponds to an aggregated conformation of the translational release factor eRF3, which suppresses nonsense codons. [PSI+] modifies cellular fitness and induces several phenotypes according to the genetic background. An elegant series of studies has demonstrated that several [PSI+]-induced phenotypes occur as a consequence of decreased translational termination efficiency. However, the genes whose expression levels are controlled by [PSI+] remain largely unknown. Here, we show that [PSI+] enhances expression of antizyme, a negative regulator of cellular polyamines, by modulating the +1 frameshifting required for its expression. Our study also demonstrates that [PSI+] greatly affects cellular polyamines in yeast. We show that modification of the cellular content of polyamines by the prion accounts for half of the [PSI+]-induced phenotypes. Antizyme is the first protein to be described for which expression of its functional form is stimulated by [PSI+].