Ribosome-associated quality control mediates degradation of the premature translation termination product Orf1p of ODC antizyme mRNA.

Decoding of OAZ1 (Ornithine decarboxylase AntiZyme 1) mRNA, which harbours two open reading frames (ORF1 and ORF2) interrupted by a naturally occurring Premature Termination Codon (PTC), produces an 8 kDa truncated polypeptide termed Orf1p, unless the PTC is bypassed by +1 ribosomal frameshifting. In this study, we identified Orf1p as an endogenous ubiquitin-dependent substrate of the 26S proteasome both in yeast and mammalian cells. Surprisingly, we found that the ribosome-associated quality control factor Rqc1 and the ubiquitin ligase Ltn1 are critical for Orf1p degradation. In addition, the cytosolic protein quality control chaperone system Hsp70/Hsp90 and their corresponding co-chaperones Sse1, Fes1, Sti1 and Cpr7 are also required for Orf1p proteolysis. Our study finds that Orf1p, which is naturally synthesized as a result of a premature translation termination event, requires the coordinated role of both ribosome-associated and cytosolic protein quality control factors for its degradation.