Global profiling of nascent chain interactors reveals TRIM25 as a co-translational E3 ubiquitin ligase

Nascent polypeptide chains emerging from the ribosome engage a range of co-translational factors at distinct phases of translation. These co-translational interactions are crucial for proper protein biogenesis and quality control pathways to maintain protein homeostasis. Hence, the systematic identification of these co-translational interactors provides insights into how distinct polypeptide fates are determined. Here, we developed nascent-chain interactor profiling (NCIP), a metabolic-labeling- and chemical-crosslinking-enabled proteomics method to identify proteins interacting with nascent polypeptide chains at a proteome-wide scale. Results from NCIP support the co-translational assembly model of multiple protein complexes and reveal TRIM25 as a co-translational E3 ubiquitin Ligase. TRIM25 ubiquitinates misfolded nascent chains for quality control at the ribosome. Our results provide a generalizable framework to systematically profile co-translational interactors.