Mechanical forces regulate the composition and fate of stalled nascent chains

The ribosome-associated quality control (RQC) pathway resolves stalled ribosomes. As part of RQC, stalled nascent polypeptide chains (NCs) are appended with CArboxy-Terminal amino acid tails (CAT tails) in an mRNA-free, non-canonical elongation process. The relationship between CAT tail composition (alanine [Ala] and threonine [Thr] in yeast) and function has remained unknown. Using biochemical approaches in yeast, we discovered that mechanical forces on the NC regulate CAT tailing. We propose that CAT tailing initially operates in "extrusion mode," which increases NC lysine accessibility for on-ribosome ubiquitylation. Thr in CAT tails prevents the formation of polyalanine, which forms α-helices that lower extrusion efficiency and disrupt termination of CAT tailing. After NC ubiquitylation, pulling forces on the NC switch CAT tailing to an Ala-only "release mode," which facilitates NC release and degradation. Failure to switch from extrusion to release mode leads to the accumulation of NCs on large ribosomal subunits and proteotoxic aggregation of Thr-rich CAT tails.