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

摘要

As translation proceeds, the nascent polypeptide chain passes through a tunnel in the large ribosomal subunit. Although thisribosomal exit tunnel was once thought only to be a passive conduit for the growing nascent chain, accumulating evidencesuggests that it may in fact play a more active role in regulating translation and initial protein folding events. Here we havedetermined single-particle cryo-electron microscopy reconstructions of eukaryotic 80S ribosomes containing nascent chainswith high a-helical propensity located within the exit tunnel. The maps enable direct visualization of density for helices as well asallowing the sites of interaction with the tunnel wall components to be elucidated. In particular regions of the tunnel, the nascentchain 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.