Visualising nascent chain dynamics at the ribosome exit tunnel by cryo-electron microscopy


One of the important questions in protein synthesis on the ribosome is how the nascent polypeptide chains fold during translation. Addressing this question has strong implications to health and disease. Ribosomes play an essential role in maintaining a healthy cellular proteome by modulating co-translational folding. And folding of the nascent chain (NC) is likely to be initiated within the ribosomal exit tunnel. Here, we report high-resolution cryo-EM structures of stalled ribosome nascent-chain complexes (RNCs) at two biosynthetic translation time-points that examine the role of the ribosome during co-translational folding. Structures of the RNCs reveal that NC is highly dynamic and adopts a range of trajectories within the vestibule of the exit tunnel, affecting positions of the folded immunoglobulin domain outside the ribosome. Local rearrangements of several ribosomal components suggest key sensor checkpoints that monitor co-translational protein folding.