Protein folding, a process that underpins cellular activity, begins cotranslationally on the ribosome. During translation, a newly synthesized polypeptide chain enters the ribosomal exit tunnel and actively interacts with the ribosome elements -the r-proteins and rRNA that line the tunnel -prior to emerging into the cellular milieu. While understanding of the structure and function of the ribosome has advanced significantly, little is known about the process of folding of the emerging nascent chain (NC). Advances in cryoelectron microscopy are enabling visualization of NCs within the exit tunnel, allowing early glimpses of the interplay between the NC and the ribosome. Once it has emerged from the exit tunnel into the cytosol, the NC (still attached to its parent ribosome) can acquire a range of conformations, which can be characterized by NMR spectroscopy. Using experimental restraints within molecular-dynamics simulations, the ensemble of NC structures can be described. In order to delineate the process of co-translational protein folding, a hybrid structural biology approach is foreseeable, potentially offering a complete atomic description of protein folding as it occurs on the ribosome.

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. ribosome | nascent chain | co-translational folding | cryo-EM