Absence of nascent peptides triggers nonfunctional ribosome decay

The nonfunctional rRNA decay (NRD) pathway eliminates defective ribosomes to maintain protein synthesis integrity. Although the Mms1-Rtt101 E3 ligase is known to trigger 25S NRD to clear defective 60S subunits, the specific molecular marks and targets it recognizes remain unknown. Here, we demonstrate that the late-stage maturation factor Reh1 functions as a specific molecular sensor for these defects. While Reh1 is normally displaced from the polypeptide exit tunnel (PET) during successful translation initiation, it is persistently retained on nonfunctional 80S ribosomes incapable of peptide bond formation. This prolonged retention, driven by the absence of a nascent peptide, provides a physical mark that recruits the Mms1 complex via Reh1’s N-terminal domain, triggering site-specific ubiquitination of ribosomal protein Rpl19 (uL6). Our findings reveal that the cell repurposes a maturation factor as a sentinel to monitor translation competence, achieving robust detection of diverse ribosomal and maturation defects through a single molecular pathway.