rRNA expansion segments mediate ribosome dimerization as a conserved stress response.

Inhibition of messenger RNA translation is a common feature in proteostatic stress cellular responses. Puromycin, a widely used compound for studying translation, disrupts protein synthesis by mimicking the 3' end of aminoacyl-transfer RNAs. Despite its extensive use as a research tool to probe the connection between translation activity and various physiological and pathological states, the cellular response associated with puromycin-induced translation stress remains incompletely understood. Here, we used electron tomography and topology analysis to define the effects of puromycin on the translation machinery in situ. We show that puromycin-treated neuronal cells exhibit an accumulation of eIF5A-bound ribosomes in a translationally inactive "idle" state, and thereby defining a broader role of eIF5A in ribosome homeostasis. Additionally, the idle ribosomes formed dimeric complexes mediated by ribosomal RNA expansion segments, suggesting an evolved mechanism involving these regions in translational hibernating and protecting idle ribosomes. We further show that the hibernating disome formation is not unique to puromycin administration but represents a conserved mechanism as a response to different cellular stressors including endoplasmic reticulum stress and amino acid depletion. Collectively, our findings illuminate distinct states of mammalian ribosome hibernation and dimerization, providing new insights into the relationship of cellular stress and the dynamic regulation of ribosomal activity.