Ribosomal RNA expansion segments mediate the oligomerization of inactive animal ribosomes

Cells down-regulate protein synthesis when stressed to conserve energy and shift resources toward repair. We found that in some mammalian cells, including neurons, stress also resulted in the formation of inactive ribosome-ribosome clusters (disomes). We used cryo-electron tomography (cryo-ET) to visualize ribosomes in situ and observed that this ribosome dimerization was mediated by a homotypic interaction of the ribosomal RNA (rRNA) expansion segment ES31Lb. ES31Lb interactions were both necessary and sufficient for disome formation and conferred a growth advantage and stress resistance to brain cells. ES31Lb is predicted to homodimerize in ~20% of chordates, including variants in both chicken and human. Cryo-ET analysis of chicken tetrasomes revealed an interaction between ES31Lb and ES9La. Thus, in animal cells, translation regulation can use a flexible component of the protein synthesis machinery-rRNA expansion segments.