Threonine-rich carboxyl-terminal extension drives aggregation of stalled polypeptides

Ribosomes translating damaged mRNAs may stall and prematurely split into their large and small subunits. The split large ribosome subunits can continue elongating stalled polypeptides. In yeast, this mRNA-independent translation appends the C-terminal alanine/threonine tail (CAT tail) to stalled polypeptides. If not degraded by the ribosome-associated quality control (RQC), CAT-tailed stalled polypeptides form aggregates. How the CAT tail, a low-complexity region composed of alanine and threonine, drives protein aggregation remains unknown. In this study, we demonstrate that C-terminal polythreonine or threonine-enriched tails form detergent-resistant aggregates. These aggregates exhibit a robust seeding effect on shorter tails with lower threonine content, elucidating how heterogeneous CAT tails co-aggregate. Polythreonine aggregates sequester molecular chaperones, disturbing proteostasis and provoking the heat shock response. Furthermore, polythreonine cross-seeds detergent-resistant polyserine aggregation, indicating structural similarity between the two aggregates. This study identifies polythreonine and polyserine as a distinct group of aggregation-prone protein motifs. [Display omitted] •tRNA levels influence the composition of C-terminal extensions of stalled polypeptides•Threonine-rich extensions form detergent-insoluble aggregates•Threonine-based protein aggregates display robust seeding effects•Polythreonine aggregates sequester polyserine When ribosomes stall during translation of defective mRNAs, the resulting incomplete polypeptides form detergent-insoluble aggregates. Chang & Yoon et al. demonstrate that threonine residues drive this aberrant protein aggregation in Saccharomyces cerevisiae. This study uncovers a distinctive protein aggregation mechanism.