Collision-induced ribosome degradation driven by ribosome competition and translational perturbations.

Individual stalling of catalytically inactive ribosomes at the start codon triggers ubiquitination of ribosomal protein uS3 and subsequent 18S rRNA decay. While collisions between ribosomes during translation elongation represent a more widespread form of translation perturbation, their impact on ribosome stability remains unknown. Here, we clarify a bifurcation in ubiquitination-mediated ribosome turnover, identifying a collision-induced branch of uS3 ubiquitination and small subunit destabilization in yeast. This pathway eliminates not only non-functional ribosomes but also translationally active ones with a prokaryotic-like decoding center, driven by competition with wild-type ribosomes due to differing translation rates. We further show that endogenous ribosomal subunit stoichiometry shifts toward a small-subunit-shortage state via ubiquitination upon perturbed translation triggered by the anti-cancer drug cisplatin and the growth phase transition. These findings reveal a mechanism by which ribosome dynamics generally affects ribosome stability, implicating ribosome dysfunction, heterogeneity, and stress-related translational disturbances in small subunit degradation.