α-Helical nascent polypeptide chains visualized within distinct regions of the ribosomal exit tunnel

Abstract

Previous studies argued that nascent polypeptide chains can form secondary structure in the ribosome exit tunnel despite spatial constraints. Using single-particle cryo-EM reconstructions of eukaryotic ribosomes carrying nascent chains with high helical propensity, density consistent with helix formation is now observed in the exit tunnel as are interactions with tunnel proteins. As translation proceeds, the nascent polypeptide chain passes through a tunnel in the large ribosomal subunit. Although this ribosomal exit tunnel was once thought only to be a passive conduit for the growing nascent chain, accumulating evidence suggests that it may in fact play a more active role in regulating translation and initial protein folding events. Here we have determined single-particle cryo–electron microscopy reconstructions of eukaryotic 80S ribosomes containing nascent chains with high α-helical propensity located within the exit tunnel. The maps enable direct visualization of density for helices as well as allowing the sites of interaction with the tunnel wall components to be elucidated. In particular regions of the tunnel, the nascent chain adopts distinct conformations and establishes specific contacts with tunnel components, both ribosomal RNA and proteins, that have been previously implicated in nascent chain–ribosome interaction.