Protein products of non-stop mRNA disrupt nucleolar homeostasis

Summary Mutations that cause ribosome stalling or impair the cell’s protective response to stalling have been demonstrated to cause neurodegeneration, yet the mechanisms underlying these pathologies remain poorly understood. Here we investigated the fate of defective proteins translated from stall-inducing, nonstop mRNA that escape ubiquitylation by the Ribosome-associated Quality Control (RQC) E3 ligase LTN1. We found that nonstop protein products accumulated in nucleoli and this localization was driven by polylysine tracts produced by translation of the poly(A) tail of nonstop mRNA. Nucleolar sequestration increased the solubility of invading proteins but disrupted nucleoli, altering their dynamics, morphology, and resistance to stress. Changes in nucleolar morphology are consistent with a simple physical model in which LTN1 impairment enhances the inter-molecular interactions of nucleolar components. Our work elucidates how failure to degrade the protein products of stalled translation may affect distal cellular processes and will inform studies on the pathology of neurodegenerative disease.