Deciphering the dynamics of divergent co-translational folding and assembly pathways

At the critical intersection of synthesis and folding, the ribosome is emerging as a hub, guiding the emergence of the polypeptide-chain into the crowded cytoplasm. The ribosome orchestrates elongation rates as well as co-translational binding of folding chaperones, protecting the nascent chain from aggregation. A recent breakthrough from our work revealed that even the final step of folding, the assembly into complexes, is coordinated with translation. However, mechanistic characterization of co-translational assembly pathways remains a major challenge, as diffusion, recognition, and binding are all coupled to the dynamic processes of synthesis and folding. In order to capture and characterize the dynamic pathways of complex assembly, we combined selective ribosome profiling, super resolution imaging and extensive molecular dynamic simulations. Our results demonstrate co-translational assembly pathways can evolve in order to rescue subunits accumulating destabilizing mutations, protecting them from aggregation. Using dynamic conformational sampling on several complexes, we identified interface “hot-spot” residues, deriving the exact point during synthesis allowing for a meta-stable complex to form. These predictions directly correlate to the observed onset of co-translational association, demonstrating our ability to predict protein-protein association at the nascent-proteome level. Imaging analysis by smFISH of the translation process in living cells, utilizing super resolution approaches, revealed co-localization of mRNAs encoding for specific complex subunits, in dedicated cytoplasmic foci. Together, our research provides crucial mechanistic insights on protein synthesis, folding and association pathways, opening new horizons for therapy of aberrant protein assemblies, characteristic of numerous conformational diseases.